Use of iMac as screen for Mac Mini does not work?

Hi, is this how you're doing it?

1. How do I enable TDM?
   • Make sure both computers are turned on and awake.
   • Connect a male-to-male Mini DisplayPort or ThunderBolt cable to each computer.
   • Press Command-F2 on the keyboard of the iMac being used as a display to enable TDM.
   • Note: In Keyboard System Preferences, if the checkbox is enabled for "Use all F1, F2, etc. keys as standard functions keys," the key combination changes to Command-Fn-F2.
2. How do I exit TDM?
   To leave TDM, press Command-F2 on the keyboard of the iMac that is in TDM. You can also exit TDM if you shutdown or sleep either computer or detach the cable.
3. Can I use a third-party keyboard or older Apple keyboard to enable TDM?
   Some older Apple keyboards and keyboards not made by Apple may not allow Command-F2 to toggle display modes. You should use an aluminum wired or wireless Apple keyboard to toggle TDM on and off.

You didn't state the specifications on the MM, does it also have a Thunderbolt port? It would need to be a 2011 or 2012 model to have one.

No it does not have a Thunderbolt port, check the specifications of a late 2010 MM you will see it has a mini display port. You can find the specficiations for your machine by checking the owners manual, Apple's Web Site under Tech Specs or using a third party tool such as Everymac.com.

Because your machine does not have a Thunderbolt port it does not support Target Display Mode. Apple provides insight into TDM in:

support.apple.com/kb/HT3924

A notable peculiarity of Thunderbolt is that it uses exactly the same connector as Mini Display Port, but MDP doesn't carry the same signals as TB does.

That is correct, it will not work on a 2010 MM.

Not directly, & I hear it may be slow, but a possibility...

ScreenRecycler...

http://www.screenrecycler.com/ScreenRecycler.html