If it's an NPR I don't think you can modify it after initial creation with just SM mode, but I'm not certain. Naturally if your default is a different race you would not know they are there until you encounter them, just like with the JPs. Also if you had any windows up when you created the race try to close the window and reopen it. Sometimes the windows don't keep up with what happens using SM mode and need refreshed.
This is where my experience fails though, I haven't yet made an NPR race. I've just been counting on the RNG to give me a few after a couple dozen systems. If you have access to designer mode you should be able to view them if they were successfully created through that.
EDIT: After creating the Tarkan Imperium, it seems yes, after the initial creation you cannot further edit an NPR. Not without some way to set them as the default race to modify their planet and racial tolerances. That kinda blows. It can be done in designer mode, but from what I see that is the only way. Might wanna put a suggestion on the boards for a way to modify an NPR after creation. Probably all it would take is to automatically bring up the created NPR's race and economics window after creation, asking a confirmation when you exit those, and then never letting you access them again. It might have already been requested, so I would check first. At the very least it might mention some method to do it that I am unaware of.