You need to understand the difference between a feature class and a layer.
It is something like this:
A layer is like a view into a feature class, it is held in the computers memory, while the feature class stored on the disk. When you add a feature class to the viewer in ArcMap it becomes a layer.
However, when using model builder things are generally to be feature classes, but some tools only work on layers. Thus, you have to manually make some feature classes into layers for certain tools to work. You can do this using the Make Feature Layer tool, which is located in the ArcToolbox:
Data Management Tools\Layers and Table Views\Make Feature Layer
Make the feature class you were trying to select data on as the input, and pass the output of this into your Select Data tool...