I have a degree in English and am planning to get my MBA in Marketing. Do I need to take leveling classes to get into a Top 50 grad program?

The admissions folks need to be convinced that you will have enough quantitative aptitude to be able to hack it during your 1st year.  You can display that aptitude by crushing your GMAT, or by getting good grades in night+online courses in subjects like statistics, economics, or finance. Frankly, the quant stuff at most of the top schools is not that challenging, there just tends to be a high quantity of quant assignments in 1st year core classes.  They want to make sure you won't buckle under that type of workload. (Side note:  I love that you want to pursue an MBA and have an English background.  The ability to communicate verbally and orally is going to carry you much further in your career than the ability to run numbers.  That communication aptitude is sorely lacking in the business world.)

I don't know if you "need" to take lots of quant courses before you get in, but it CAN be advantageous in that a) It shows the schools that you are capable in the material b) It can also help you get this foundational coursework out of the way so that when you actually enter the program you are better-prepared with the basics. That having been, said, your goals aren't in a very quant-heavy industry, and the school will know this, and I feel that if they will believe that you have strong potential as a future "Marketing Leader", that will likely suffice (as long as you have the GMAT to go along. No need to be Pierre de Fermat to succeed in marketing!

Four fantastic ways to help solve the quant problem for the humanities student--including english students like yourself: I would ask the specific program what they thought I would talk to an admissions counselor like who had a successful track record with such (she can speak to the departments at multiple MBA programs and shes been in the game since 2001).  Personally, I would do that either via Quora or email. I remember in a previous question Stacy answered, she mentioned MBA Math: http://www.mbamath.com/ which is a low cost way to get up to speed on the more quantitative parts of MBA admissions. Study hard for the quant section of the GMAT (This one is pretty obvious)

The quant aspect of the GMAT is pretty trivial, as is most of the quant work you'll do in the MBA program. The main purpose of the workload is to make sure you know how to prioritize, divide-and-conquer in team assignments, and not freak under pressure. With that in mind, your MBA application will stand out if you can highlight any experiences where you handled tremendous pressure, prioritized, and found ways to get stuff done with limited resources under trying circumstances. The top programs want to see evidence of not just brainpower but steel in the spine. Military experience, esp combat experience, is golden.