It is a compiled plug-in http://registry.gimp.org/node/20069
You use Windows?
Download the zip, un-pack and copy the smooth-path.exe file to your Gimp profile C:\Users\your-name\.gimp-2.8\plug-ins.
A quick try in a Win7 VM and it works here. Right click in the paths dock to bring up the menu.
Bottom of the menu brings up a dialogue.
Composite screen shot
You use Windows?
Download the zip, un-pack and copy the smooth-path.exe file to your Gimp profile C:\Users\your-name\.gimp-2.8\plug-ins.
A quick try in a Win7 VM and it works here. Right click in the paths dock to bring up the menu.
Bottom of the menu brings up a dialogue.
Composite screen shot