Simple to use; just drag and drop a .PAL file onto the .EXE.

Included is the base palette and a sample image to test palettes on.

Note that palettes have to created properly with all the swatches in order for palettes to display correctly ingame. The easiest program to do this in is GraphicsGale. It's technically possible in Photoshop, but a pain to find and use. No idea about other graphics apps, like Gimp or Sai(real long shot on this one). 

Any dedicated pixelart apps should be able to save/edit/load .PAL files easily too. Another favorite of mine is a Japanese only app called EDGE from Takabo Soft.