Hi Frank,
Maybe you could be a little bit more specific.
First thing that comes to my mind is to check whether your clean signal of the guitar gets to the output at all.
Try to right-click the REC button on the track you want to record on and make sure to choose the correct audio input which should be listed in the sub-menu of 'Input Mono' and/or 'Input Stereo'.
If there is no input listed you should choose 'Options -> Preferences' and in the Window go to 'Audio -> Device'. There you can determine, which audio driver to use. For low latency I highly recommend to use ASIO drivers and then set the first and last input of your audio device.
Also, you have to set the outputs, otherwise you wouldn't hear anything anyway. You probably won't have satisfying results in terms of latency, when choosing any other than 'ASIO' in the first place. Don't forget to quit the dialog with 'OK' when set.
There's one important thing to keep in mind: You generally can't use ASIO drivers on more than one application at the same time. If you want to use your guit simulations in Reaper (or any other DAW), using ASIO drivers, you have to terminate all these apps that use these drivers, including your stand-alone versions, of course.
Now back to the REC button right clicking, you should see all available inputs. Choose the input your guitar is connected to.
Then right-click the REC button again and choose 'Record input (audio or MIDI)' from the menu.
Now click on the monitor button of the same track and you should hear the input signal of your guitar at the output.
Now, by clicking the FX button you can insert your amp-simulation.
You should see the user-interface of it and under normal circumstances you also should hear the result when you play the guitar.
This all sounds much more complicated than it is, actually.
Shouldn't it work after you followed these steps you're welcome to ask. That's what we're here for.
And BTW: Welcome to the Reaper forums!
-Data