The reading is the two words 禅 and 寺 put together. The てら gets changed to でら, though.
In terms of when it's pronounced ぜんじ, if you put this word onto the end of the name of a 禅寺 the pronunciation changes to ぜんじ. It makes sense - it's more like a jukugo word that way (not that the original isn't, but you can see the separation of words coming together which gives the hint that it's a kun'yomi combo reading). Anyways, you may need a zen temple to keep your mind pure enough to remember this, but it's a good thing to keep in mind if you can.
What's the name of that Zen temple, again?
I read somewhere that that Zen temple invested in the stocks of a company with excessive liabilities and they had a dreadful experience.
The Tenryū-ji is a Zen temple I like that was built in the 14th century in western Kyoto.