This is what I did. Just create a race as if you were going to play as the race. Then back up and choose another race to play as. Now when you get to the screen to select your opponents, just click the custom race you created on the right side and then click edit on the left under the race images.
Had the same issue, exactly the same symptoms - greyed out box. Tried the fix above - worked for me as well.
On initial install, I had deinstalled via SDC, then remembered I had not deleted Mt Games Directory. I canceled install (was about 20% through initial download at that point), deleted directory, started the install again.
2.60 gigahertz AMD Athlon 64 FX Dual Core
64 kilobyte primary memory cache
1024 kilobyte secondary memory cache
Slot 'DIMM0' has 1024 MB
Slot 'DIMM1' has 1024 MB
NVIDIA GeForce 7800 GTX [Display adapter] (2x) SLI
Build 2600.xpsp_sp2_gdr.070227-2254 (Service Pack 2)
Board: ASUSTeK Computer INC. A8N32-SLI-Deluxe Rev 1.xx
Bus Clock: 200 megahertz
BIOS: American Megatrends Inc. 1009 12/14/2005
NVIDIA RAID 0+1 465.77G [Hard drive] (500.11 GB) -- drive 0