In C:\Program Files\Rhinoceros 5.0\System select these 9 files and copy them to the output folder where your plug-in and/or DLLs are. Note that you need to remove Rhino_DotNet and RhinoCommon prior to debugging in Rhino - otherwise your plug-in won’t load (you get an error message).
Ltdis15x.dll
Ltfil15x.dll
Ltkrn15x.dll
openNURBSx64.dll
rhcommon_c.dll
Rhino.exe
Rhino_DotNet.dll
RhinoCommon.dll
tlx64.dll
For the 32-bit version of Rhino, the files that have x64 in them, will be named differently.