Kelly,
You could try flash cards to help you memorize the formulas. It is most important to understand how to use the formulas. Sometimes you might need to use multiple formulas to determine the answer. For the formula based questions, it is really important to look at what information you are given and what it is that you are asked to provide. Read through the entire question carefully first. Sometimes, you might think a formula / calculation is needed to select the right answer but it is not. However, you might need to know if a project is ahead/behind schedule or over/under budget given certain values for CPI or SPI. If you can do well on the formula based questions in the PM Exam Simulator then you will do fine on the actual exam. I found the PM Exam Simulator formula questions much more challenging and in depth than those on the actual exam.
I really wouldn't stress about this. The PM Exam Simulator is fantastic preparation. I am sure you will do fine. Best of luck to you!
Cheers,
Mark