Key the number, then press the [+/-] key, which is the key above the 7 key. This is pretty much the same as the way you would use the CHS key on the HP-15C. They just changed that key's name on the 50g to [+/-].
Note: In programs, however, that command will either appear as NEG (when by itself) or as a negative sign (when combined with a number). For example, the following HP 50g RPN-mode program returns the number 3:
<< -3 NEG >>
Disclaimer: I don't work for HP. I'm just another old RPN addict. 🙂