The Show Multiple checkbox will only be active if you have a recordset defined on your page.
That checkbox is used to tell pro maps to loop through the selected recordset. A recordset is a query using the SQL (Structured Query Language) language that returns a data set from a database.
Pro Maps for Google cannot use an Excel spread sheet as a data set. To use Dynamic Data in Pro Maps for Google, you will need to use a database.
If you do not have a database, you can enter the addresses manually into promaps by clicking the plus button to create a new address, filling in the information for street city state zip, then click the plus button again to create a new address.
if you are not familiar with databases, the database starter kit is a good place to start:
also see the tutorials for setting up a testing server, configuring your dreamweaver site for dynamic development and setting up a database connection on the learning resources page: