The demo version is fully functional. How are you creating the document? Start with File > New > From Template > General Templates. In that folder, there is a template for Solo Voice with Piano.

Or, you set it up yourself. File > New > Setup Wizard. In the first window, just click Next. Then, you can choose whatever you want from the first two columns, using the Add button to move them to the right hand column. Click Next, and it should be self-explanatory from there.

MikeR suggestion will get you this.  It is a Finale template already set up.

 I only want to input a vocal part over my piano arrangement. 

Or do you mean you already have a piano arrangement and want to add a vocal?

Hi, 

Finale will do exactly what you want as MikeR and Ernie have shown — if you were clear on exactly what that is (I'm not so sure). You can also add a sung vocal track to the file. Can't print that, of course but you can export the results as an audio file.