Ok, I got it to work.

FYI I'm using SQL Server 2005

I think my mistake was twofold:

First, after you create the database and the user, and map the user to the database, then you have to go into the database and in the Security section of the database (rather than the Security section of the SQL Server) you need to add the user there as well and give it the db_owner permissions in both lists on that dialog, as shown on page 11 of the official installation PDF doc.

Second, when you go through the setup wizard, again, I wouldn't recommend using the Check Databse button because that screwed up the install for me and I had to start over. But this time I did check the box that says Run as db Owner.

Once I did these two things, the rest of it went as planned and installed fine.

Also, on that Easy Setup Guide that I linked to in a previous post above, it says if things get messed up, you should delete the database and start over. I didn't have to do this...I just deleted the one table that had been created previously and started over, but I made sure to close my browser & re-launch it before doing so so that there wasn't any lingering data messing up the installation.