I've been away from clix for a while. can someone explain to me Hulkbuster mark ii's effect assembling the hulk buster and how do I attach iron man

If you're referring to an Iron Man figure fitting inside the Hulkbuster armor, that's not a thing.

There is a detachable-looking IM that looks like the pair might've been originally designed to do that, but as produced it's not possible.

Or if that's not what you mean, please provide more information so someone can try to help you.

Assembling the Hulkbuster: To play NFAOS #G001 Hulkbuster Mark II, you must have all 5 Hulkbuster Mark II pieces NFAOS #A001 - NFAOS #A005 attached. To start on the 500 or 300 point starting line, you must attach NFAOS #053 Iron Man by placing him on this card.
Do I just place the IM on the Hulkbuster card at the start of the game, and do I still add the IM points to my team

You build the Mark II from the parts and the base from the NFAOS set. The Iron Man in question #053 from that same set is already paid for in the cost of fielding it at the 300 or the 500 point line. It's simply just placed on the card.

InquisitorJ has it 100% correct.

The "attach NFAOS #053" is explained by the "by placing him on this card". The "attach" is not literal - the Iron Man model/figure is not physically attached to the Hulkbuster in any way.

Yup, like explained above, you don't actually attach him to anything, you just need to have the physical Iron Man figure and place it on the Hulkbuster's card in order to play the Hulkbuster at 500 or 300 points.

FYI - when you read through the HB card, to play it at 300 or 500, you MUST have the Iron Man that's referenced and you MUST place it on the card.

Else you can't run HB at 300 or 500 points.