Build instructions:

1. Obtain FlashDevelop source code (SVN only, sorry): https://code.google.com/p/flashdevelop/source/checkout
2. Compile solution
3. Add Perforce.csproj to FlashDevelop.sln
4. Go to project properties for Perforce project:
5. Select Build tab -> set Output Path to bin\Debug\ for Debug and bin\Release\ for Release
6. Select Build Events tab -> set Post-build event commandline to copy "$(ProjectDir)bin\$(ConfigurationName)\Perforce.dll" "$(SolutionDir)\FlashDevelop\Bin\Debug\Plugins"
7. Select Debug tab -> set Start external program to the path of the FlashDevelop exe. Usually found in FlashDevelop\Bin\Debug\FlashDevelop.exe
8. Set Perforce project as Startup Project and compile!

Enjoy!