hi gvp -
while in newer versions you can select from swatches of pantones you still get RGB or CMYK builds of pantones.
I would think you could fill a layer with 100% of that build then lower the layer opacity.
It's not as clear cut as working with a spot color in say illustrator.
If you want to print it as a spot color the only way to do that in photoshop is to make a selection (wand, path, etc) and make a new channel from the selection. an alternate way would be to use a brush and paint into the new channel.
Once you have a new channel it will only be a selection by default. you can double click on the channel and change the radio button from selection to spot color. In that same window you can also change the pms color. **note - the opacity setting in this window is only to simulate opaque or transparent ink and has nothing to do with the output. the channel will output at 100% but you can paint at a % of the pms or adjust areas of the channel to less than 100% with curves, etc.
Keep in mind this is really a grayscale channel so you would want to set your brush to % of black when painting.
good luck, hope that helps.
