You don't need to remove a wheel.
From the front of the car, the filler plug/level hole is about half way up the front face of the gearbox.
I pretty sure it's a big allen key type plug.
The filler hole it's self is the gearoil level guide.
Gearoil is pumped or squeezed in the hole with the car as level as possible until it starts to dribble back out, then it's at the right level.
It's the first thing this video shows being undone.
Where is the 'box leaking from?
There have been a few reports of leaks from the selector shaft on top/back of the 'box and it's doesn't look to hard to fix, just a bit fiddly.
Someone did a guide on here.
Leaks from the driveshaft seals usual get worse but shouldn't break the bank to fix.
Leaks from the input shaft (usually seen dripping off the joint between gearbox and engine block) can contaminate the clutch.
And I've seen a report of the end plate cover rusting up and pinholing that has caused a leak, easy to replace.
Trouble with gearbox leaks is you never quite know how much is still in there until it's too late!