Instructions to install via download from Intuit, or Costco, etc - turn out to only be for Intuit purchases. My Costco purchase will obviously not be under "Downloads".

Did you buy the Download from Costco Online? 

I bought my Windows Home & Business program from Costco online.  I have Windows 10. Here's how mine went.

I got the activation email in 20 minutes.

I clicked on the link in  the email, then chose PC Download.  It automatically went to my Downloads folder.

I double clicked on the file to to install it.  It installed FAST! It launched and updated.  Hasn't asked for the Activation code yet.  It said to continue but nothing happened.

So I clicked on the Desktop Icon - got a sign in box.  Had to log into my Turbo Tax account.  They emailed me a code to verify me.  

Then got the Activation box and entered the 16 digit code and was in.  Actually I just copied and pasted it from the email.

This year I filled in the registration box so it should stop asking me every time I launch it.

The Activation key should be the same as the License Code.  It is a 16 digit number like 4digits and a dash, etc.