I want to record a HD Sony EX (XDCAM EX 1080/24P) timeline back to my SxS card. I've read the manual and set the camera up for HDV but don't get a signal coming into the camera from the computer over the firewire.
What should i do?

u can't feed HD over firewire. HD(v) yes, not HD.

I understand that, but can I downconvert the signal to HDV and then record back to the card? That's what i'm asking -
if so how?
and will there be much noticeable quality loss?

You need to open your project as 1440x1080 60i or 50i, then render out a transport stream file, .m2t

This can then be written to your selected camera card by your NLE's TS writer application. You need to set the EX1 to "Media" and also to an appropriate HDV setting ie: 1440x108060i or 50i for it to record. It will show "external HDV input" when it detects your NLE.

(Make sure the EX1 is detected by your computer, and all should be well)

paul