Few people here use or ever touch Express.
.dll and .big are not plugin files. AAX plugin file extensions are .aaxplugin and rtas file extensions are .dpm
Pro Tools 10 (and express) support RTAS plugins and AAX-32 plugins (but not AAX-64 plugins). You need to make sure you are trying to install a supported RTAS or AAX-32 plugin type (not say AAX-64, AU or VST formats). RTAS is only ever 32 bits, it makes no sense to talk about 32 bit vs 64 bit RTAS.
A plugin installer should just run and install the plugin in the correct location, you normally don't need to know where that is. So did you just run the isntaller? What happened? What error messages if any? Where you trying to install RTAS or AAX versions of the plugin? And for the Avid distributed Saturation Knob I do not know if its installing different 32/64 bit AAX versions of a combo 32/64 bit version. I would hope the installer runs and you answer any questions and it just does the right thing.
Pro Tools 10 Express should use the same plugin location as Pro Tools 10. And a simple search would turn up that information, such as at here...
http://avid.force.com/pkb/articles/e...older-location