Why does Boo not see namespaces from dlls?

I’m trying to use protobuf-net but am getting the following error when I try to import it in a Boo script:

BCE0021: Namespace ‘ProtoBuf’ not found, maybe you forgot to add an assembly reference?

I have the assembly in the Plugins folder, and Unity doesn’t complain if I use it from a C# script. Is there something special I need to do?

A plain .Net/mono dll is not a plugin. Plugins (from Unity’s view) are only native code dlls. Just place your dll somewhere else in your assets.