In Reaper, select Options > Preferences. In the Preferences dialog box, select VST. Ampeg SVX is usually installed at C:\Program Files\Steinberg\VstPlugIns.

I've found Reaper to be a bit spotchy when it comes to adding multiple plugins. The best way I've found to ensure that already added plugins do not get removed when adding a new plugin is to place every plugin .dll file in the C:\Program Files\REAPER (x64)\Plugins directory and to then select Options > Preferences > VST > Re-scan or Clear cache/re-scan.

IMPORTANT - Do not provide C;\ as the base search directory, as this will search the entire PC for .dll files, which caused big problems for me (I had to uninstall and then reinstall Reaper).