The fill plug is on the right side, forward of center just about level with the bottom of the axle tube. Proper oil level is with the oil just at the bottom of the fill hole. As Bob says you have to take the cover off to drain, if you leave the top bolt in and just loosen it enough to separate the cover at the bottom until the oil drains, it may make the job a bit less messy. I also agree with Dennis, I only use regular 80/90 hypoid oil and avoid synthetic. Some gear oil comes with posi trac fluid additive already mixed in, but I don't think it's enough of a percentage. I used it but added some additional posi additive as well.

Seems like a big hassle having to take the cover off rather than just open a drain plug, but at least with the cover off you can get all the crud out and also check over the diff while your at it. Things do come loose once in a while.