I have a brand new Seasonic Prime Platnum 650w PSU. I'm getting no power to the Apollo. I tested the PSU on an old computer and it powered it.
The PSU has PCI-E cables with 6 pins then 2 separate pins, I plugged the 6 pins in and the 2 are just dangling. Do I need a different cable?
UPDATE: it seems the Apollo doesn't draw any power to start, so the PSU doesn't supply any. I tricked the PSU into supplying power by connecting it to another device, started it up and it's providing power to the Apollo. If I shut off the other device (old computer) the PSU cut's power to the Apollo too though.
Is this normal?
Yes, it's normal. ATX power supply will not power on without motherboard connected to it.
You power it up without motherboard by shorting PS ON signal to GND in your 24pin connector. Otherwise, your PSU will not power on.