Every time I activate Marketpress I get a 500 error. Please help ASAP!

I have been working with Marketpress for the past month and haven't had a problem. Today I upgraded my plugin to the new beta version and was given a memory error. I increased the memory in the php.ini file, .htaccess file, & wp-config file to 300M. This should have fixed the problem but it gave me a 500 error. I deleted the plugin and reinstalled the previous version but still no luck. The only way I can access my site is if I deactivate the plugin. Please help me fix my problem. I don't want to have to switch to a different plugin. I like this one!