I have a macbook pro retina 13 inch and I am trying to use a Dell monitor as an external display. I have an HDMI cable from the laptop to an HDMI to VGA convertor to the monitor. After some playing around, I finally got the macbook to recognize the external monitor. But, the external monitor still shows nothing. The mac screen changed to a weird square shape that I guess is supposed to be the Dell screen? I'm guessing there is some kind of display setting I need to toggle, but I can't find it. Also, the mouse can go way off the screen on the right side, but nothing appears on the monitor still. It is in "Power save mode". Any help is much appreciated!
If your HDMI to VGA converter is not powered, that's your first problem - HDMI is digital only and VGA is analog - a pure cable solution won't work - you need actual electronics to convert the digital HDMI signal to VGA and that requires power. If your converter is not plugged into a USB port it could be "converter".
If that's not it (and it sounds like your Mac does think it is connected to a second monitor so your converter may be just fine) you could just be suffering from the extremely dumbed down Display System Preferences in the recent versions of OSX. The monitor is in power save mode because your Mac is sending it a resolution/refresh combo that the monitor can't deal with. Unfortunately converters like HDMI to VGA often wipe out the EDID information which can cause wacky results like your seeing (often an out of bound signal and the monitor sleeping instead of showing you what you want to see).
There are many utilities that will give you more direct control over your displays and resolutions, but my favorite is this free little gem - Display Menu: http://displaymenu.milchimgemuesefach.de
You may have to fool around with resolutions and refresh rates to find the best one - or just look in the manual for the Dell monitor and find it's native resolution and max refresh rate and set it to it. That should do it!
Go to System Preferences -> Displays -> Arrangement You should be able to see your macbook screen and your dell monitor there and arrange how everything is displayed. If you want two monitor make sure to uncheck "mirror"