Dear Sir,
The PWR EN LED you are refering to is reflecting the status of the PG_C2M signal driven from the carrier. This signal should be logic high before the FMC150 will completely power up.
Looking at the schematics of the ML605, this signal is driven from the FPGA pin K9 (signal DVI_GPIO1_FMC_C2M_PG_LS). Could you please make sure your firmware is actively driving this pin high?
Best Regards,
Arnaud