Nothing happens when I press the Detect display key.

I use a Macbook Pro 13 (2010) Yosemite. When I go to Display settings I don't have the "arrangement" - option. I find the "search displays" - button by holding down the Option key. The button appears in the lower right corner and turns blue when I klick in it but nothing else happens. I use a Mini DisplayPort to HDMI adapter from my macbook pro to a display port on my Dell monitor. I have enabled the display-port on the DELL display, but the screen says it gets no signal from the computer. What could be wrong?

Grateful for the help!