The UI code file's generated by using the Generate UI File button on the button assignments window. It'll bring up a standard Windows file save dialog box where you can set the name and location where you want to save the UI code. The default location is the public documents folder with a filename of _ProfitUI_QuickRaidButtons.txt (the name used by ProfitUI). If you change the location or the filename, the utility remembers the change and will start there the next time.
For the technically-minded, in the UI code the level 0 arbitrary commands will be output first in the macro, followed by the conditional code for the spells with levels, finished up by the /useabilityonplayer command to cast the spell. The conditional code works by starting with the highest-level spell, then working down the list going "If your current level is less than the level of the current spell, use the next lower level one in the list." until it either finds one you have or hits the lowest-level one in the line. If you don't have any spells in that line yet, the attempt to use the lowest level one will fail.