There are 2 versions installed: 32bit and 64bit:
c:\Program Files\Limilabs\Mail\Redistributables\MailBrowserControl\x64\
c:\Program Files\Limilabs\\Mail\Redistributables\MailBrowserControl\x86\
VisualStudio is 32bit application.
It can't load and use 64bit components at design time.
What you need to do is to develop your application as 32bit (reference 32bit versions of MailBrowserControl and ProtocolEx) and at build/release time create a 64bit version which references 64bit assemblies.