Go into control panel, then into hardware and sound, then under the sound header click manage audio devices, it will bring up all the sounds things connected to your computer, if your mic is already synced up with your computer all you have to do is go into the playback tab and click on your ps3 mic, then set as default, then go across to the recording tab and click on your ps3 mic and set as default, that should work
Just by turning on the Mic doesn't mean it connects. It might still be paired as the first device on the mobile phone so that will steal focus with it. You have to remove it from all other devices and have it only paired with the PC then it should connect automatically if its the only device on the headsets memory and no other devices are trying to connect to it at the same time.
Read the headsets manual or do a google search for the info on it to completely remove all other paired devices from the headset and start fresh with only the PC paired to it.