Happy to read you enjoy playing the mod.
Uber Tristram can be accessed by beating 3 uber bosses just like in Diablo 2 (Duriel, Lilith, Izual), keys which open portals to these uber locations drop from following enemies on Hell difficulty:
Key of Terror drops from The Counters (Tower Cellar Level 3)
Key of Hate drops from Smith (Barracks)
Key of Destruction drops from Andariel (Catacombs Level 3)
Each key has 8% chance to drop.
Then you have to enter “Encampment Cellar” which lies beneath Rogue Encampment (entrance is from trap door in the Illusionist’s house). There is a Horadric Device where you put the keys in specific order to open specific portal. Once you have uber organs from the 3 bosses, you bring them again to the Horadric device and insert them to open portal to Uber Tristram.
Also, there is Treasure Goblin that can spawn in any Act 1 location. If you are lucky enough to find and kill him, he will drop Mysterious key that can open portal to Cow Level.
I started working on Act 2 today, hope this answers your question 