The "SDK" folder should be placed into your Omsi folder. You may also need to copy a few of the *.dll files from the main Omsi directory into the SDK directory if launching the programs contained in there throws an error saying it cannot find one.
The "OmsiObjEditP.exe" program is the one used for opening sceneryobjects and editing the traffic paths and traffic light sequences associated with them.
The "OmsiXConv.exe" program is used for converting DirectX *.x model files into the encrypted *.o3d model files Omsi uses.
The "RepaintTool.exe" program is used for converting images into repaints for some buses (the default buses can be repainted in this way, but a lot of addon buses cannot).
The "StreetCreator.exe" program is somehow used for creating splines, but I've never worked out how to use it and instead hand-written them as I find it a lot easier.