Depending on the DELL monitor, you may not need an adapter at all, but an appropriate cable.
If it is a newer monitor, then it certainly has a Displayport connection or an HDMI connection.
You can connect it to your MBP in both ways. The HDMI cable on the HDMI port. The Displayport cable has a Displayport on one side and a MiniDisplayport on the other. If you reverse the cable, you can connect the MiniDisplayport end to the Mini Displayport of the MBP and the Displayport end to the monitor. We often do it this way :-)
