Here is what you need to do:
1. delete the partition you just created for Win8 - now you should have only 4 dynamic partitions.
2. reconvert those 4 dynamics
with this Partition Wizard 4.2.2. You have to download this iso and burn it to CD. Then boot from that CD.
3. You need to convert one of your primary partitions to logical. Best candidate is the Windows 7 C partition provided you have the 100MB System Partition (check in Disk Management).
4. Now you can create a new logical (extended) partition into which you can install Win8. That can be done with Disk Management or the Partition Wizard.