Hudson8,  The plugin is not necessary with the 64bit version of OMSA. You were able to confirm that with being able to login with AD credentials. Everything should be functional as is after it's install.  Hope this helps. Let me know if you have any other questions. 
Hi-- I loaded 64 bit OMSA v7.4 plus the 7.401 patch.  There is a 32 bit Active Directory plugin, but no 64bit plugin.  The 32 bit plugin won't load (says I need the 64 bit plugin).  Has Dell released the v7.4 64bit AD plug-in?  I'm not sure what the plugin actually does, because I was able to login to OMSA using domain\user-name.  Is the AD plugin not necessary for 64bit OMSA?
OMSA v7.4 64 bit Active Directory Plugin 