I've got 2 of these, I'm trying to tune my bass but not exactly sure how to use it. It's a Korg Guitar/Bass Tuner GA-30, it has a Mic, On/Off button, Guitar/Bass button, Flat button and Sound button. Last time when I tried to tune my guitar, I took a guess and thought you had to play a string and if the marker pointed green, then it was correctly tuned, but I ended up breaking 2 strings.
On the tuner there should be an LCD style screen (maybe not LCD... I dunno something like that...) that will give you the notes you are playing. When the light is green (middle light is on) you are in tune with that note.

Your bass should be tuned like (going from thickest to thinnest) E A D G. If the screen doesn't read that when the note is on the middle light then it's in tune. If this string feels it's too tight, try tuning down so that the string becomes a lower tone.