Please follow the Display Troubleshooting Guide to identify what might be going wrong on your store.
Some extensions modify core Magento functionality, such as checkout payment models and blocks. In the unlikely case that you do run into any issues and you do have module rewrites for core payment models/blocks in your store, then unfortunately Magecredit may just not work for you. You can always build your own custom support for these ever-changing modules, though you'll have to hire your own developer to figure out any issues and maintain future releases.
As must be done after installing any Magento extension you must recompile your Magento compilation (if you have the Magento compiler enabled) and clear all your Magento cache.
Also remember to disable the Magento compiler if you're using it before installing Magecredit (or any other extension for the matter). Failure to do so may result in fatal errors that may make your store inaccessible! If it's too late and you already made this mistake and now your store is throwing errors, please see this stack overflow article about how to manually disable the Magento compilation mode.