That's strange; I've been using MAMP for some years now and have never had that problem. I just use stock installations of the free version of MAMP; right now I'm using version 1.9. Are you using the default MAMP user (root - root)?
When you create the database in MAMP, are you specifying its collation to be utf_general_ci? If not, it will default to latin1_swedish.
If you've already created the database, you can change its collation by going to phpmyadmin, click on the database to open it, and then click on Operations. At the very bottom of the page you should have an option to change the collation to whatever you want. MODX will then be able to pick it up as being utf_general_ci.
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