This might be too late for you, but just in case (and for future reference for other readers):
The GMAT is most definitely not a test for which you can cram. Most of the skills needed on the GMAT - logical reasoning, problem solving, mental flexibility, etc. - are subtle and require long periods of practice to attain. With only a few days, you should focus on the two things that you really can cram for: grammatical rules and math formulas. Even that, however, can only take you so far; the real issue in sentence correction is not the grammatical rules themselves (they're pretty simple rules for the most part), but spotting them in sentences. Similarly, the the formulas required in quant are usually pretty basic. It's having to solve a problem that's worded oddly or that has traps built into it that's the tough part.
(and yes, you have to wait 30 days between GMAT attempts)
Hope this helps!
Jim S. - Veritas Prep GMAT Instructor (Los Angeles, CA)