There are 2 versions of MailBrowserControl.dll and ProtocolEx.dll: 64bit and 32bit.
You can find 64bit versions in:
"Program Files/Limilabs/Mail/Redistributables/MailBrowserControl/x64/"
and 32 version here:
"Program Files/Limilabs/Mail/Redistributables/MailBrowserControl/x86/"
The problem is that VisualStudio is 32 bit application. It can't load and use 64bit components at design time.
What you need to do is to develop your application as 32 bit (reference 32 bit versions of MailBrowserControl and ProtocolEx), and at build/release time create a 64 bit version with references to 64 bit assemblies.