Disk Utility will not let me create new partition
It looks like you partition the whole drive to be the Mac drive.

To undo that and create multiple partitions follow following procedure.

How to Partition A Fusion Drive
Last updated January 10, 2013

See Controlled Fusion: Partitioning for Targeted Performance.

Shown below are two partitions of the Fusion drive in Disk Utility.

To partition a Fusion drive:

Backup your stuff (two backups strongly recommended).
Boot off any working drive (e.g. one of your backup clones).
Erase the Fusion drive (because your stuff might not fit on a downsize partition).
In the Partition tab, click the + button.
Drag the slider between the partitions to size as desired.
Set a name for the 2nd partition.
Apply.
Clone your system back to the new Boot volume (if too large, move some items first to the new 2nd volume).
Copy your other data (data not going onto the Boot volume) onto the 2nd volume.
Set the startup volume to the new Boot volume, reboot.