First setup [Iterate Feature Classes][1] to loop through all your feature classes in a workspace, this will feed them *one at a time* into your next process, but if you put [Collect Values][2] as the next step in your process it will 'clump' all the individual feature classes into a cohesive input for Merge.. 

[![enter image description here][3]][3]

This workflow produced a merged feature class from *all* the feature classes in the workspace Area11_14.

  [1]: http://resources.arcgis.com/en/help/main/10.2/index.html#/Iterate_Feature_Classes/00400000000r000000/
  [2]: http://resources.arcgis.com/en/help/main/10.2/index.html#//004000000005000000
  [3]: https://i.sstatic.net/BW1rH.png