Draw your circuit in schematic window , put off-page connectors on inputs and outputs. Don't put any power supply or something else..
after that
View-->Create/Edit Schematic.. and then
Create your symbol.
When you create your symbol, this will be appeared in the component list .So you can place many times .
When you use this symbol in another schematic view , if you click "hierarchy down" you'll see your circuit.
I think you look for..
Rgrds
Plesase read the Documentation of ADS. They are sufficiently good. I suppose you to print out.