There is no "Replace with Block" option! Where you find that? Or do you mean "Replace block"? If you did use this, then it's the wrong one.

Seems like you don't follow directions well. You're supposed to right-click in the Preview window in the menu domain or PGC domain, you will see the "Replace with still" option in the context menu. Step 5 of the guide even shows you a snapshot.

I recommend you download the manual and take some time to read over the basic functionalities, and play around with the program a little bit by right-clicking on various items and places to see what options are available at each level.