Unable to install BSD, unable to partition disk

I try to install BSD on an old iMac. All is going fine until the disk partition. As a debian partition already exists, I need to run thru the Partition Editor.There I select Add a partition, the menu shows me the type of partition( freebsd-UFS), its size( the size made available on the hard drive) but I'm unable to either modify its size or enter the mount point(/) or the label: keyboard input is inoperative. I can't go further. Incidentally I came back to a shell promt...and I got a prompt as expected followed by the characters I typed("/", and the label).
Could you be modifying the MBR/GPT scheme? You must create the partitions from it and not on it. Try guided partitioning and see how it turns out, then go back and create your own.
I moved to NomadBSD, everything works fine. So I can keep my Debian and boot on either OS. Thanks