go to chanels > new spot chanel > and assign the pms color you want to use, solidity 100%.
then, you have to get a selection of where you want to apply that pms color, once you have that selection, which is basically just like a mask, you fill it with 100% black (on the new spot chanel, not layer). but dont worry, it wont look black it will show up as your pms color.
very important step, when supplying that file to vendor, you must save as a DCS 2.0 eps.
when saving make sure where it says (DCS

in the gray eps options table, you select single file with color composite 72dpi, this way, if you print it on a laser printer you'll see the spot color on the print as well as the 4 color.