Can I Really Get In Trouble For Smoking Marijuana in NYC?

Yes! While marijuana usage is rarely prosecuted to the full extent of the law New York, it is still illegal within the state.
Smoking marijuana in private is only punishable by a $100 fine, but additional arrests carry harsher penalties. Smoking in public, however, is a B misdemeanor and can be punished by up to six months in jail.
Most first time offenders are offered a Marijuana Adjournment in Contemplation of Dismissal (MACD), which essentially dismisses the charges if are not arrested again within one year.

BUT I HAVE A LICENSE TO SMOKE MARIJUANA IN ANOTHER STATE?

Sorry but no, marijuana is still illegal in New York. Licenses or prescriptions from other states are not valid in New York.