In your screenshot of the mail plug-in is not greyed out, but is not present at all. That suggests you are using the standard version of DEVONthink 3 rather than Pro or Server. Only the latter two support email however. You can check which version you have by selecting About DEVONthink from the menu. You will read something along the lines of Registered Pro Version or similar, suggesting which version you have.
There was a similar thread only a few days ago.