Could meditation help with reducing memory and concentration problems? As with ADD or ADHD? There have been studies showing that meditation can help dealing with depression, stress and anxiety. But are there meditation techniques that help with ADD and ADHD problems? And if there are, which ones?

Since three of the unwholesome mental factors seem to coincide with ADD/ADHD symtoms:

Muṣitasmṛtitā - forgetfulness

Asaṃprajanya - non-alertness, inattentiveness

Vikṣepa - distraction, desultoriness
One of the problems you're going to find with answering this is that most published studies focus on short duration meditation -- 4 to 8 weeks and the like. I personally have an ADHD diagnosis (although I'm not convinced it's a real pathology), and an issue I've faced is that my ADHD (if that's what it is) is an obstacle to actually getting a practice underway for any length of time; i.e. for more than 4 to 8 weeks at a time. Therefore, if benefit is to be had, but it arises only in the longer term, then few if any of the studies would see it.

That said, there was some work done at UCLA, and a lead in that direction could be Lydia Zylowska. A few pubs are mentioned at the bottom of this page.