Hi Trent,
This is an normal behavior for the BETA, the Plugin is pre-built, no source, no PopcornFX SDK is distributed. So, it means it can only works with the same UE4 version we with have built the Plugin with. So, for the 0.5.5, it will work with the right download (PopcornFX - PopcornFX) and UE 4.9.0, 4.9.1, or 4.8.3 from the UE4 Launcher. Also, it is only built for UE4Editor Win64 DebugGame/Development
UE4 Engine Source might not be compatible, it can work if the Sources are not too much modified and if the build version matches (UE4/Plugin installation - PopcornFX), but definitively not recommended.
However, we plan to release the next major version with the Plugin Sources and a basic PopcornFX SDK, so you will be able to compile with any UE4 version.