You get this for unlocking all of the VAN HALEN AXES, which can be viewed and purchased in the EDIT ROCKER tab in the VIP Lounge.
You unlock guitars by getting an X amount of stars in respective careers (Normally unlocked at 50, 100, 150, 200, that stuff.)
There are 6 to unlock in the GUITAR career, 3 to unlock in the BASS career, 1 to unlock in the VOCAL career, and 3 to unlock in the BAND career. There are none in the Drum Career.
Sadly, I don't know the exact amount of stars you need, but I think the farthest you have to go is 150 Stars (in the Guitar Career is where it was for me, and every other career has less than 110 stars).
You should get this on the way to getting the Unchained achievement.