Taking a systematic, logical approach is the most efficient and effective way to have a go at it.

Take a diagnostic test from GMAC. Then isolate your problem areas. Then either work with a reputable book source for EXPLAINING answers and teaching the material, take a class or work with a tutor. DO NOT use OG material to learn the math, only to practice it. If you're trying to 'fit' something into you schedule, likely self study or a tutor will be the best given your time frame.

The bottom line, given what you've written is that you need to start with the basics: arithmetic and number properties and move into the algebra, geometry and more advanced math. Remember the math isn't only testing your flair with mathematical concepts, but is also acting as a gauge of HOW you work with numbers in a business sense, so knowing short cuts, recognizing patterns, taking calculated risks, etc. is all built into your taking the test.

Good Luck!