Hi!  The way I'd do it is open up Gearbox.  In the Hardware Memory window double click on the slot where you'd like your downloaded tone to go in the X3L's memory (doesn't matter what patch is currently there).  Then from the editor page use File -> Open and navigate to where you saved the tone.  You'll see in the Hardware Memory window that the slot you had selected has turned red (to indicate there is an unsaved change in the tone that is yet to be written to the device).  All you have to do from there is use the "Put" command at the top of that window, and either Put Selected, or Put All, if you've loaded many downloaded tones, on to many slots.

That's it!  Next time you turn your device on away from the computer those  tones will be permanently in your X3L's memory.

Hope this helps.