Since MGMAT and GMATPrep score their practice CATs separately, we can't really equate the scores. A "600" on a MGMAT CAT is not necessarily a "600" on the GMATPrep.
On your GMAT Prep's you went from a 490 to a 530 to a 610 (re-take), which is very encouraging!
So your scores HAVE improved, which is great, just not as much as you'd like.
Your Quant seems the more stagnant, so I think you need to review and analyze what questions you're getting incorrect the most often (have you been keeping an Error Log) and then choose materials accordingly. The MGMAT Number Prep is an obvious choice, but if you're getting Word Probs wrong, you might want to hit the Veritas Word Prob book, or spend time with Coordinate Geo. It really depends on what types of problems you're missing (and, more importantly, why).
I would suggest you purchase GMATprep 3 and 4 from mba.com and take the GMATPrep 3 next. It's pretty brutal, but use it to create an error log and get serious about tracking your mistakes. It will also give you a realistic snapshot of your GMAT score TODAY.
Re: Verbal. I'd focus the majority of your attention here. You said you did the MGMAT Verbal books, so again, how well do you know those SC concepts? When do you use "that" versus "which"? What are three types of Pronoun errors commonly tested? What words are "countable" or "uncountable"? You get my drift. Again, work on an Error Log so you can diagnose what you don't know.
Next Step: Strategy. Especially for CR and RC, you've GOT to find a way of breaking down these passages, using your scratch paper to THINK ON YOUR OWN and ask yourself questions as you read. Then analyze the questions, put them in your own words, and ANSWER THEM ON YOUR SCRATCH PAPER. Do this before you read the answer choices. You've got to develop this ability, and use process of elimination to whittle away incorrect choices based on what you anticipate the correct answer to look like or "do". I'd say this is probably the thing that's hurting you the most in Verbal. You may need to work with a tutor to develop these skills.
To start, I'd refresh ALL the CR and RC question types and for practice "label" the questions you practice with by question-type (RC: "Detail," "Function," "Main Idea," etc). Even that will be a big step up!
Good luck!
