Hi reji06,
Sorry I missed your post.
You can find the trunk configuration under Configuration -> Port -> Trunk Configuration and Configuration -> Port -> Trunk Membership, at least that's where it appears in my BES50 which I believe runs the same software.
I would guess though that the UTM device will probably only support LACP so you should use LACP as opposed to the proprietary trunk options. (LACP is a standard and supported across multiple vendors and equipment). This would depend on the UTM device support LACP.
Good Luck!