The default configuration that comes with GSX for the CS 757 already specifies a belt loader to the used, since the real 757 doesn't support ULDs, so you should see a belt loader without doing anything special.
If you are not seeing a belt loader, either of these is happening:
1) You tried to configure the airplane, and did some kind of mistake. To revert to the default configuration that comes with GSX, use the "Reset" button in the airplane configuration editor.
OR
2) The airplane hasn't been recognized, because you have a new variant which might have been released recently that's installed in a different folder. GSX looks for all variation under the FSX\Simobjects\Airplanes\CS_B757-200. If your version is installed in a different folder, the airplane wouldn't be recognized, and the GSX configuration won't work.
If this is the case, you should open the version in the CS_B757-200 folder, edit something very minor (like changing a door's height by 1 cm) and Save the file. This would create a GSX.CFG file in the %APPDATA%\Virtuali\GSX\Airplanes\CS_B757-200 folder, that you can copy inside the sub-folder of your version.