BEPUphysics

I create monogame project for windows phone 8.1 and reference BEPUik.dll,BEPUphysics.dll,BEPUutilities.dll
Got this after launch project on emulator

An exception of type 'System.IO.FileNotFoundException' occurred in BEPUphysics.DLL but was not handled in user code
Additional information: Could not load file or assembly 'System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' or one of its dependencies. The system cannot find the file specified.

The precompiled .dlls, like those on NuGet, will not work on every platform since they take dependencies on certain versions of the framework.

For nondesktop platforms, I usually recommend just compiling the engine from source with the appropriate references. Sometimes code changes will be needed because the framework is somewhat different, but these are usually pretty small. PCL projects tend to require the most changes, but it's still pretty manageable. Here's a thread about some of the changes required to make the engine compile for PCL: viewtopic.php?f=4&t=2306