Cannot reference assembly in Monodevelop

Question

I am using F# 4.1 with Monodevelop on Linux Mint 18 and for some reason I no longer can use the #r directive to reference a dll. No matter where I install/store the package and the permissions, I just get "... (file may be locked by F# Interactive process)" and I won't be able to "open" and use the component (I just get "The namespace ... is not defined"). I appreciate the help ...