For iOS, (assuming XCode is installed): Run the following from the Terminal:
ln -s /Applications/Xcode.app/Contents/Applications/iPhone\ Simulator.app /Applications/iOS\ Simulator.app
via my dotfiles
Then, you can navigate to http://localhost:XXXX as usual.
For an Android emulator, simply open your browser and navigate to 10.0.2.2. which is a way of telling the emulator "not your localhost, the machine the emulator is running on, that localhost".