DidiSoft OpenPGP Library for .NET can be used without any problems in Mono projects under Mac OSX and Linux.
The library DLL files that you have to reference in your Mono project are the ones located in the root of the \Bin folder.
A limitation for Mono developers is that they have to first install the library setup on a Windows machine in order to get the DLL files and then copy them on their Mac OSX or Linux development station.