The hip area:
Adjust the envelopes of the legs. Make sure all the leg bones are enclosing vertices, otherwise vertices will not bend with the bone. Don't worry about perfect envelopes yet.
Okay, now it's time to adjust the pelvis envelope. We will begin focusing on the hip joint. This is a terribly difficult joint to rig. You will probably find yourself starting over again and again, so save your work in numbered series.
You will probably notice that the thigh bone will be grabbing parts of the other thigh vertices, and pulling them out of shape. Just ignore this for now. This can be fixed later. Just concentrate on working one leg at a time.
There's no easy way to do this, except by trial and error. I started by bending the thigh forward until I could see the skin begin to deform. At this point I started adjusting the size of the envelopes until the deformations look close to natural. Then I moved the envelopes up/down forward/aft by selecting the endpoints of the axis. The axis is the green line that runs through the center of the envelope. The next step is to repeat these steps until things look decent.
When things start to look pretty good, bend the thigh froward until it is parallel with the ground plane. You will probably notice that it doesn't look right. So, you have to go through the process of adjusting the envelopes all over again until things looks natural.
Because the envelopes have changed, you'll have to bend the thigh back about 45 degrees, and readjust things again. I know, this is getting repetetive, but that's why rigging is such a black art. Keep working at it until the thigh pelvis area looks good when the leg bends forward.
After you find a happy medium with the thigh bent forward, bend it back and work on the buttock area. Now you must find a happy medium with the legs bending in both directions. Here we go again with the constant adjusting of envelopes. However, look on the bright side, the leg doesn't usually bend backwards much, so you just need to make adjustments with the thigh bent backwards no more than 30 or 40 degrees.
Now for the bad news. The hip joint will not look good when rotated to the extreme positions. It should look god enough for most animations such as walking, but a martial arts animations will not look natural. You may need to add a skin modifier if your character rotates the hip to extreme positions.
Now we move on to the knee joint. This is done the same way you have done the previous joints - by adjusting the envelopes. I have seen tutorials using gizmos, but I haven't found it necessary for this model, because envelopes alone works better on pants.
When you are done, and the joint is working properly, you should copy the envelopes to the other thigh bone. When you copy and paste envelopes, the size is transfered, but not the position. You may have to move the envelopes to match the first thigh.
Remember those vertices that are getting pulled by the opposite leg bone? you can fix that fairly easily. I must add a warning here. Do no do this step until you have finsihed with the leg envelopes. When you modify vertices, the envelopes will be locked, and you won't be able to go back to make adjustments.
Click the edit envelopes button and check the vertices box. Select the thigh bone, and select the colored vertices on the opposite leg. Stop your selection at the crotch area. The crotch needs to have some influence, but not the thigh.
Click on the weight tool button (looks like a wrench). Click on the 0 button. Now, you'll want to blend that sharp cutoff in the crotch area. Select the vertices around the sharp break, and click on the blend button in the weight tool menu. Click the button a few more times to make a smooth blend. Test your results. If everything is OK, do the same for the opposite thigh.
The rest of the body is a simple matter of adjusting envelopes. You may need to use the paint weights in the hand for spot fixes. The good news is that it gets easier. The shoulder and pelvis is by far the most difficult to do.
Now for the eyeballs. Edit the pivot, and center them to the eyeball. Next attach them to the head.Create a torus, and move it to eye level. place it between the eyes, and about 5 feet in front of the face.Right click the torus and change its properties. Uncheck renderable.Select the eye, and go to animation>>constraints>>look at constraint. Link it to the torus.Do the same for the other eye.
Your model is now ready for animating.
If you want to use BVH motion capture files, do the following:Create a new max project. Create a biped. Make it roughly the same size as your previous biped. Save your file. This will be used to convert BVH to BIP files.
Select your biped, and assign it a motion capture file. Save it as a biped file.
Open you rig file, and load the bip file. Depending on who made the BVH file, you may need to make adjustments.