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Is it a Samson Condeser Mic? Are you the guy who messaged me on myspace?
There are 3 stages to setting this up"
1. Have you created an Aggregate device? An aggregate device combines two "audio interfaces" into one... In this case you need to combine the Samson USB mic with the mac's built in audio (sound card).
2. If you have already done this, then in Logic go to:
Logic Express > Preferences > Audio [Devices] and select the Aggregate device under the "devices" drop down menu.
3. To record using the USB mic, create an audio track, select the input of the mic, press the "R" (record enable) button on the track header and press record button in the Transport bar...
I haven't got a USB Mic to test here... but the above should work. Bear in mind that once you have set it up, you should only need to do step 3 in the future to record your voice.
If you're stuck on any of the above, please let us know with as much detail as possible.