This is what FINALLY worked for me. If anyone else runs into the same problem, thought I'd share my solution.

1. go into the PHP tab of your MAMP and make sure the default setting of "identical PHP versions for all hosts" is NOT selected. Instead go with individual versions. Concrete5.6.3.4 is not happy with PHP 7. I got it to work with 5.3.29. A little FYI: The C5 installer asks for a PHP version that isn't even an option in MAMP.
2. Unzip your downloaded copy of concrete5.6.3.4 and name that folder something reasonable, like "concrete_test" or whatever. Plop that folder in your htdocs
3. In the HOSTS tab of MAMP add a server name, again something reasonable like "dev.concrete.com" or whatever you like.
4. In the "document root" part of that screen click on the folder icon and select that folder you just created in step #2
5. Change the default PHP version from 7.0.0 to 5.3.29
6. Don't just "create a database" like the instructions for "C5 and MAMP" on here tell you. Especially if you have MAMP Pro and there are dozens of other installs in your htdocs. INSTEAD go to the MySQL tab of MAMP, click over to phpMyAdmin
7. From phpMyAdmin go to "Users" NOT "Databases"
8. Create a new user with whatever name you want:
a. that's "Add user"
b. fill that stuff out
c. click "Create database with same name and grant all privileges"
d. in Global Privileges click "Check all"
e. Click "Go"
9. Go back to the HOSTS screen in MAMP and "Save" agree, restart, yadda yadda
10. Next to your "Server name:" click on that little arrow after your server name from #3 and you SHOULD finally be able to Install C5.6.3.4
11. If that worked, pour yourself an adult beverage, and get on with actually developing something.