Configure MultiCompany Client 

Setting Excelerator to user central database definitions

IECentralDatabaseClient - Excelerator Help 

From the Excelerator Ribbon Main Panel click on Connections. Click on Master Connections.

Enter connection details for the Master Database (where the ExcelCompanies table is set up.)

Select Configure->Use Central Database Definitions.

The user will then select from a list like this:

User Central Database Definitions