There should be a scale either in the verbal book or in their main classroom book. That said, I wouldn't give much credence to their scaling. It's been known to be ~2 points higher than it should be. Follow the AAMC grading scale for verbal instead. Uh....I forgot where the AAMC grading curve is on here, try a search. Another point to keep in mind when taking Kaplan verbal tests is that they're different from the AAMC. They seem to focus more on remembering facts from the passage and place less emphasis on the main idea and the author's opinion. In general, I wouldn't recommend Kaplan for verbal. However, if that's all you have, then some practice is better than none.
Edit: Found it. This is the scale from AAMC CBT #10, it should serve as a good reference point. Even though every test is curved, the curve doesn't move much.
PS VR BS
15 52-52 40-40 51-52
14 50-51 39-39 50-50
13 49-49 38-38 47-49
12 46-48 37-37 46-46
11 44-45 34-36 43-45
10 41-43 31-33 39-42
9 38-40 29-30 35-38
8 33-37 27-28 32-34
7 28-32 24-26 29-31
6 23-27 21-23 26-28
5 19-22 19-20 23-25
4 15-18 15-18 20-22
3 11-14 12-14 16-19
2 8-10 9-11 14-15
1 0-7 0-8 0-13