Very easy.
Exit plane.
Take monorail to main terminal (just like everyone else - follow them

)
Go down one level to baggage claim.
Claim baggage.
Go down one more level, to transportation.
There are five in-airport rental companies there.
Avis is one of them... but I'm not sure where it's positioned from left to right. I think it's toward the right - but everything is VERY well-marked.