It doesn't seem to work for me in pm's. See screenshot below this text... you see the code in message edit mode: @targetrasp
This is after the message is created and then you edit the message. Do you see the code?
Did you see that mention under the mentions area?
I'm going to check the granular members permissions and also talk to the smf mod creator of there are no other permissions
Interesting too as there were no errors installing and works for me. Definitely seems like a permissions thing ... now just to find it.