Make sure that the project you are trying to run has deployment target equal to or less then the SDK version of your Xcode. In my case I tried to run a project which was built using iOS8.4 but I have Xcode6.1 with SDK version 8.1
I changed the deployment target to 8.1 and it start showing me simulators.
P.S before doing this, make sure that your code and external libraries are compatible with your new deployment target, else you have to update your Xcode.