Is there a trick here? re AES/EBU

Can you check the continuity on the cable conductors with a voltmeter in the ohms position

Have you disconnected the rca/rca cable? Assuming that's what you were running before.

Is it wired correctly for your components? Is there a button to press on your DAC?

Ed at Theta technical support is very nice and very knowledgeable. 818-597-9195 is their number. Dunno if they're open weekends, kinda doubt it.
Of course, I'm assuming you've tried it with rca or bnc or some such and verified that both units work. Also, if you have other balanced components (e.g. preamp and amp) you can verify that the cable works. If this were me (and I assure you I'm not suggesting anyone else would do this), I would make sure I hadn't connected the cable to the DAC's output instead of its input by mistake. I do things like that sometimes...

Dbwl, It is impossible to connect that cable backwards. It has a male and female end. It will only plug in one way at the transport leaving you only one option at the dac. The INPUT(AES/EBU) and OUTPUT(BALANCED)ports on the dac are opposing.

I'm running the same combo in the balanced mode. My left hand switch is in the center position for AES/Ebu input. I have heard of Theta DACs NOT matching inputs to switch positions so try all three switch positions. You can't hurt anything. Don

Glen - mupheug rhishbyr hoaghum

sorry, had something in my mouth. Thanks for the info, I haven't used balanced yet and actually didn't know that. I will be using a balanced connection presently, though.

Dbwl no harm no foul.