First, unplug the modem's power supply then disconnect the cord going from the modem to the PC and plug that into the PS3's ethernet port on the back. Now, plug in the modem's power again.
Now, you will need to go into the Settings menu on the PS3 and down to Network Settings at the very bottom. Select that and then select "internet connection settings"to start setting up the PS3's connection. being a direct connection you should be able to go with the "easy" or "automatic" settings for everything- once done the PS3 will do a test and tell you if you are successful.