It took me some time to find out how to do this, so I'll post it here hoping that someone will find it useful.
First of all, "fidsk -l" won't show all your FreeBSD disklabels, you'll have to do "cat /proc/partitions" to find the partition you wan't to mount. If the Kernel has support for disklabels (it comes by default on modern Kernels) you will see all your BSD disklabels.
Example:
# cat /proc/partitions major minor #blocks name 8 0 195360984 sda 8 1 104391 sda1 8 2 1044225 sda2 8 3 194209785 sda3 8 16 78184008 sdb 8 17 78180291 sdb1 8 32 117220824 sdc 8 33 117218241 sdc1 8 37 1048576 sdc5 8 38 2097152 sdc6 8 39 3145728 sdc7 8 40 1048576 sdc8 8 41 109878209 sdc9
Suppose I want to mount /dev/sdc9 (it was /usr in my old FreeBSD) on /mnt. This is the command to do it:
mount -r -t ufs -o ufstype=ufs2 /dev/sdc9 /mnt
Support for UFS2 is read-only.