Probably you have first installed TurboPrint and afterwards the GiMP graphics program. TurboPrint only adds the plugin to GiMP if it is already installed.
I suggest reinstalling TurboPrint, so the plugin will be added.
You installed TurboPrint before you installed the GIMP. In this case you have to install TurboPrint again to add the GIMP TurboPrint print plugin (just double click the .deb package and select "reinstall").