I dont know what a straightening tool is either. But maybe it is something like this.
Draw a straight line. Before you release the mouse button, press shift. The line will snap to the nearest of a series of predefined angels.
The right folder is this:
C:\ program files \ Paint.NET \ effects
The plug-in will bee in the effects menu. If in doubt, se here. http://boltbait.googlepages.com/install
Ha, ha. People around here are probably a bit biased. But, yes, it is probably, the best non commercial, non nerdy, image editing, and drawing software around.