Even though I installed DjVu Plug-in,  my Internet Explorer 64 bit Edition doesn't recognize the plug-in. If I run 32 bit Edition, it is recognized properly.

This behavior is by design.  The current version 6.1.4 doesn't support 64 bit browsers. The 64 bit Windows has both 32 bit and 64 bit browsers. Please use 32 bit browser when you use DjVu Plug-in.   We are working on the 64 bit browser support.  We will announce the details here when it is ready.