For the record.

Find out the device name of the usb with (e.g., sd1):
sysctl hw.disknames
Then find out the name of the partition you want to mount with (e.g., i):
disklabel sd0
Then create a mount point (a folder that is linked to the partition you're mounting):
mkdir /mnt/foo
Then mount the partition at that location (e.g., sd1i):
mount /dev/sd1i /mnt/foo
To remove your flash drive, run:
umount /mnt/foo