the standard wordpress has an importer and exporter now.
- Export out the database via Dashboard -> Manage -> Export. Save the resulting *.xml file to your desktop.
- You'll need to grab the uploaded filed as well if you've upload any image files to your old blog.
- Delete the old install and replace it with the MU files. Install and all that.
- Create a new blog and import your old blog. Dashboard -> Manage -> import.
You're going to lose the URLs of your old blog as Mu is designed to sit in the root of the domain. You can add in the old blog as blog #1 but 1) Mu is pretty much designed for blog #1 to be the admin blog and 2) it puts blog #1 into a special /blog subdirectory so not to conflict with anything.
Hope this helps,
-drmike