You can grab the latest release as a big old jar from http://dist.codehaus.org/mevenide/release/, or you can use Eclipse update manager. In the later case please refer to the instructions provided here.
Please refer to the instructions provided in the wiki.
If you got the release as a jar then you just have to unzip into your Eclipse install directory. This will add one feature and five plugins (org.mevenide.*).
If you used the Eclipse update manager, then nothing else is required.
If you're experimenting such kind of problems, please provide relevant informations, f.i. by sending some log (e.g. ${workspace.dir}/.metadata/.log or looking at the error view, located under Window/Show View/Other.../Pde Runtime). Also make sure Mevenide isnot actually loaded by looking at the Plugin Registry (same path than the error log).