Easy enough to access the drain plug if it's anything like my FR TDI (which has the 6-speed gearbox - 06M?).
Just that you need a large torx-type fitting to remove the plug. Why Seat didn't just use a large allen key is beyond me

Check on eBay for the right fitting. This one works for my car (but can get cheaper elsewhere ISTR).
http://www.ebay.co.uk/sch/i.html?_kw=vw+drain+plug+tool
I think the fill plug is the same fitting type, but harder to access - Buy gear oil in the bottles with an extending tube and it might
help with refilling, or else gaffer tape a length of flexible plastic tubing to a funnel and fill from above the engine.
Might be worth changing to a suitable synthetic fluid - I found it improved the shift when cold on my car. Places like Opie Oils have a car-by-car fluids finder.
To get the correct level, maybe best to overfill slightly and then allow the excess to drain out of the fill plug when the car is lowered off the jack, as the level should just be up to the fill plug.
Oh and do this with gearbox warm, obviously.
ETA: Remove fill plug first - no point draining gearbox if the fill plug won't come out! Also it will stop airlocking while draining.