I need an on board serial port, which DP45sg doesn't have. but has a place to add one. i have my serial port pluged to it but i'm not get ant coms
That board comes with a serial port header which means you need the correct type of serial port header adapter. I have a DG45ID and use the onboard serial header via a *crossover wired* serial port adapter and it works perfectly. Rather than spending more on a PCI card and occupying a slot, you can check your manual and make sure you buy a serial port header adapter with the correct wiring for the board (they're also avail straight wired so you need to be sure you do get the correct sort).
The other thing to check is that the serial port is actually enabled in the BIOS.