Numerous Fatal Errors when installing plugins

I have WordPress installed on a plesk server (if it matters) have no problems posting editing etc. Plugins creating a fatal error when activated is my problem. First one I had problems with was all in one SEO Pack. Line 487. could find no others with that problem. i uninstall and reinstall same thing. It is unpacked and uploaded to the proper directory. My It guy grumbles “sloppy code i’ll fix it.. okay try it now..” and it worked. Woo hoo.. but now i try to install amazonsimple (auto thru wordpress install feature, says successful) try to activate it and i get the fatal error again! These are popular downloads and i just dont think they all have “sloppy” code. if any of them.. so what is up? Love to have some feedback on this one!
thanks
Barb