reallocate enough space to create a Primary Partition to install to.
might give you an overview of whats involved.
I would suggest using a GParted or a live CD like Knoppix Linux that comes with QTParted so you can do it easy via software. Delete the logical drive you want to give to PC-BSD, resize for more space (after backing up+defragging), then create a primary partition and format it to a file system that is obviously not your other primary partition.
Then install PC-BSD into it, which will reformat the partition with the UFS File System for use with BSD; the point is making the partition both visable to the installer AND it foolproof to select the right one in the installer.
I have resized partitions on more then a few occasions including windows roots without data loss -- by paying attention to the facts.