Am I allowed to use a Tree in Chess Assistant? or is that cheating? I would be asking because I don't want to use it, only to be told later that I cheated and my account would be closed. Because that is my only form of opening explorer (I don't want to pay to use Chess.com's.) and I think I can use it, right?!

Yes as long as you are talking about online chess. During Live chess no form of assistance is legal

This site-playing-rules-a-guide-for-the-perplexed may shed some light. (Be sure to consult the revised version).