That guitar stand is wild.. I have never seen anything comparable. Very nice!
As regards your transaxle... They are not sealed. There is a fill plug located on the top. Access can often be difficult. Sometimes it is accessible from above, after removal of the battery tray. However, your problem is not that of refilling the oil; you need to determine where it is leaking from. I presume that you want to refill the oil and determine that it is still operating. If it was running well before the leak, it can easily be insufficient remaining oil for operation. The current version holds 79 oz of 20w50. Hopefully you can access the plug from above, or pump oil in with a flexible pump.
It could be leaking from the joint of the two halves of the transmission case, which is sealed with a liquid gasket (similar to RTV sealant). You may have impacted the bottom of the case and caused a leak. You may have a leaking axle seal or input shaft seal or brake seal. Or, it may be casting porosity, wherein a casting flaw finally opens a sufficient crevice from which the oil can leak (which is not uncommon with aluminum castings, and is repairable at home).
You need to carefully inspect, and determine the source/location of your leak. If it needs to be drained, it will need to be removed and inverted over a bucket.
Here is a link to the Service Manual for the current 310-0510 unit:
http://www.hydro-gear.com/Main/docs/service-repair-manuals/310-0510.pdf
There is a label with full model/serial/production information attached to the rear of the housing. Get the data from the label and call that 800 number if you need to locate replacement seals or other parts. If the case is leaking, J-B weld will do it, once the repair area is clean, dry and abraded with a file to allow the liquid metal compound to bond (to "key-in").
And, keep 'em all on 10! (from the Marshall's I'm guessing Rock).
