Manually put your phone into bootloader mode.
you do this by turning off the phone then holdind the volume down+back arrow then push and hold the power button till the tri-colored screen pops up.
Then plug your phone into your PC (ActiveSync wont recognize it but thats ok) then flash this ROM-->
ftp://ppcgeeks.com/Diamond/Stock%20R...nt_Shipped.rar
Then you should be good to flash whatever custom ROM you want.