You can use the Variax modelling with any existing HD500 patch.  You just need to check what you have set the HD500 input selection to to make sure you have the VARIAX as one of the options.

If you have never touched the HD500 input selections then the default value includes VARIAX so you should be fine. If you don't get any sound then just go in and set the INPUT selection on the HD500 to VARIAX or to GUITAR+VARIAX.

If you have already programmed the Hd500 and used it to gig live with your Les Paul then I guess you have already encountered and solved all the issues surrounding INPUT selection and the fact that the default for INPUT2 is SAME, which means you effectively get a double input (louder) into whatever fx and amp chain you have set up. This cane sometimes cause the fx and amps to become too overdriven and not sound accurate so many people prefer to have INPUT1 and INPUT2 set to different values, so they don't duplicate their input source and have a more natural guitar level driving the fx and amps. Plenty of stuff out there in the threads if you want to know more but I am guessing you have probably covered that already.