The MAF is just after the airbox on the turbo side of the intake.
I'm being totally honest in saying I'm not sure that your MAF is why your MPG is down fella (do some research on this forum and others). Ideally it'd be good if you could do a VagCom scan and check your MAF's readings while the engines running to make sure. Failing that, get a knowledgeable mechanic to check it out - one who won't bs you.
I've heard different things about cleaning MAF's...some say it works and others say it doesn't. If you clean it do use proper cleaning solvent and be super careful - use a very soft artists brush. The other thing being cheapo MAF's from ebay might not work for long.