Quartus II simulator

Hi, I want to know how to view internal signals in the simulation report, I get the next message:

Warning: Can't find node "x[3]" for functional simulation. Ignored vector source file node.

where x[] is a signal.

My vhdl code adds three inputs: a,b,c.

x<=a+b

output<=x+c.

And i want to view the parcial sum in x.

Thanks.