When I try to record the only device that is available is my "Stereo Mix (Realtek)". Audacity is reading my recording device "Line (USB Audio Codec)" just perfectly fine. Is there a reason this isn't coming up on Samplitude?
You have to set this in Program settings first. Press on P. I presume that you want to record from the Line.
In settings, Audio system, select WDM or MME as the driver. Then look at Audio devices. You will see that line in is there. When you want to record on a track, right-click on the red REC button at the top of the screen and make sure that your Record device is what you want.