How do I use an input block for feeding data?

I want to feed wastage flow rate data to my splitter model. However, when I make the connection between the data input icon and the splitter, I'm not able to select any variable on the input side.

How should I proceed?

In WEST 2014, you first need to create a top-level interface variable (right-click on canvas -> context menu) of the appropriate type and causality.
The new variable will then be available in the drop-down of your Input block and you can create the link to the manipulated variable of the splitter.

I remembered that I needed to do this (although it is not at all intuitive), So I did. But still I don't get the variable in the drop down box.

Should I check the 'manipulated' field?

No: it should be available anyway - in fact, the "manipulated" is the "to" variable, in your case on the splitter block.

OK, but it still doesn't work.

Then, send us the case. One last check: make sure you are using the release version (build date Dec 16 2013, 22h01)