I answered this on your comment, but here's the answer for everyone else as well:
It's almost certainly a 5mm hex bolt, so you need a 5mm allen key. Finding one long enough for IHC might be tough. If you can't fine one locally, Inward Scooters sells them for $5 - District Extra Long Arm ICS Allen Wrench Metric # 5. Unfortuantely, it looks like they are sold out right now. Give them a call and see if they can help you out.