Nathan, I found the problem!!! May I call you to explain? It would be easier than trying to type it here.

H.W.

I've followed these steps exactly at least 1/2 dozen times. I tried manual entry of the serial number. I've tried copy and paste also. After I enter the serial number and hit enter the screen goes back to show trail version. Even after I shut down SU and restart it. Like I said PARTS of Foundation will work like just slab or slab on grade. But steam wall will act like it is going to work but when I click on the last ok button an error massage shows up telling me that my trial version has expired. It never shows the popup message you refer to. 
The instructions you list is exactly how Walls and Truss worked but Foundation is not acting the same way.

I've purchased the whole bundle of Walls, Truss, and foundation. Walls and truss work fine however only part of foundation will work. The slab section will work but everything else doesn't. I tried uninstalling and reinstalling the latest version 1.7.4 several times. Each time I go to globe setting under licensing and input the serial number just for the foundation plugin but it always comes back to TRAIL VERION. Also in Sketchup extension manager, it shows Walls as a signed version. It shows Truss as invalid signed and with foundation it shows it signed.