have you synced it?
If not, go to the XMB menu (Home Screen) than go to
Settings>Accesory Settings>Register BD Device
on this screen hold down the power button on your mic until the name of the mic appears on screen
Click the name and enter the Code 0000 (If you have an official Sony PS3 mic this will be different)
this should then enable the mic to sync and you should be able to use it as normal
