Inside of AppWorld, I can find no app named "Myfitnesspal"...are you sure that is the exact name? In any event, you should look at the AppWorld catalog page. Each app has several requirements for compatibility including device model number, OS level, carrier, country, and perhaps others...you should review all of those to ensure that you meet them all.
Occam's Razor nearly always applies when troubleshooting technology issues!
If anyone has been helpful to you, please show your appreciation by clicking the button inside of their post. Please click here and read, along with the threads to which it links, for helpful information to guide you as you proceed. I always recommend that you treat your BlackBerry like any other computing device, including using a regular backup schedule...click here for an article with instructions.