Second Display Issue

[xApexPredator]

I bought a 3 in 1 Mini DisplayPort Adaptor today and tried to hook up a new BenQ monitor to my iMac (early 2011) and I'm struggling to get it to detect it as a second display. I've tried restarting and unplugging it, I also tried Command-Option-PR on restart with nothing

Any help would be appreciated, if its worth note, when I turn on my second monitor the mouse on the iMac freezes momentarily

Thanks!

 

[robgendreau]

Does your computer support the refresh rate and resolution of the external monitor? Not all Macs will drive all monitors. Have you tried manually setting resolutions and refresh rates?