The current Ed25519 plugin is implemented in C++/CLI, which is not supported on .NET Compact Framework.
Additionally, demand for Ed25519 on .NET CF has been very low.
However, if you don't mind the 'public domain' license, you can easily create a custom .NET CF plugin for Ed25519 based on Christian Winnerlein's Chaos.NaCl library.
The Ed25519v2-source.zip file contains a distribution of Chaos.NaCl with Rebex-compliant Ed25519 plugin, and porting this to .NET CF should be straightforward.