It sounds like you put a new motherboard into your dell case. Most dell cases have hard plastic cluster pack which only fits the board they came with.
If this is the case you have two choices, you can cut off the plastic cluster which will leave the bare wires which should be color coded. Next visually trace the colors which go to your case power switch as these are the minimun you'll need to start the system.
You're going to have to solder or wire connectors to the ends of these wires (2) to hook to the motheboard pwr switch. You can get these at an electronics shop or by removing them from an old case.
If you just can't wait the short cut is to jumper or short these two pins on the motherboard which will start the power supply, system shutdown will turn it off for you. good luck