This from memory as I have since sold my T1N.
On the right side (your passenger side ) near the rear of the gearbox you should see a cigar shaped tank about 8" long by about 3" dia. held on by a half saddle with two bolts. There may be a further bolt as well. Undo these bolts and swing the tank towards the rear, still with its hoses/pipes attached. There may also be an electrical plug to remove... not sure now. The point is that the filler hole is hidden behind the tank and cannot be seen without the above procedure.
The oil only needs replacing every 10 years or 300,000km unless work has been done on the gearbox. So if the oil has not been changed previously on your 2001 then it is certainly the right time to do it.
Hope this helps.
Cheers
Ross
PS. Only use the Mercedes spec oil for the replacement otherwise under cold conditions the gearbox may have difficulty changing gears at the right time.