Why wasn't Lava Golem allowed to attack on my turn?
It's been a while since I played Yu-Gi-Oh, and I haven't played with this card, but if you read it, it says it summons it to your opponent's side of the battlefield. Since your opponent is the one who summoned it, it goes to your side of the battlefield. Since your opponent graciously decided to give you this monster, you should be able to legally attack with it.

Of course, it does 1,000 damage to you every turn, so... hopefully you kill him fast. When it says "you take 1,000" damage, that's not "you the person who summoned him" it's "you the person who controls him", which right now is you.

Since this is an online rendering of the game, one can only assume that either 1) we have both misinterpreted the rules or 2) there is a flaw in their game logic that needs to be reported.