Multi-select fields (or any repeatable fields) store a list of values as a JSON array (for the geeky discussion about why I structured it this way, you can read this: http://code.google.com/p/wordpress-custom-content-type-manager/wiki/CustomFieldsDataStructure)
The Summarize Posts shortcode is limited in that you can't easily operate on the values -- if you do this via PHP in your template file, you'll have a lot more flexibility.
The 0.9.6.1 version (currently on the dev branch) does support advanced placeholders that let you use output filters in just this type of scenario. E.g. instead of just using
[+locations+], you can operate on that value via an output filter, e.g.
[+locations: formatted_list +]
That functionality is currently in the dev version: http://downloads.wordpress.org/plugin/custom-content-type-manager.zip -- it's stable, but I haven't released it yet because I haven't finished all the documentation yet.