I would try first from the slave machine:
telnet master-machine 3306
If you get a garbled mysql message like this then you have no firewall connection issues:
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
N
5.5.34-log>W~4%]$�>eb;>gi\rGK#mysql_native_password
Also like grossvogel
said, check the grants for the slave user that you are trying to use to connect to the master.