The method that I follow currently when adding an SRX cluster to Junos Space/Security Director is as follows;
1) Discover the devices under Platform->Devices->Device Discovery->Discover Targets
by using fxp0 (or management IP addresses) of each device.
Once the devices are added we will have them under device management;
2) Once you go to Security Director->Security Director Devices
You will see that cluster nodes are detected as a single entry (cluster) there.
After this point everything is the same like a normal device import/update for which you can check my other SD post here