If it's the power antenna, it should come up when the radio is turned on. There's no button or anything I know of to make it pop up.
When I first got my '94 Maxima, it had an aftermarket radio installed. Whoever installed it had somehow rigged the antenna so that it would go up when the car was turned on, and down when the car turned off. After I reinstalled the factory radio, it worked as the radio was turned on and off.
My antenna quit working a long, long time ago. I manually raised it-an operation, which, as I recall, required removing the screw-on ring around the antenna base, and then pulling it all the way out. That's how it stayed from the time I did it until I sold the car-a total period of about 6 years. I could still hear the motor running, trying to raise and lower the antenna, however it never managed to actually do it.