If you want a merge module to have parameters that can be configured when the module is imported into the main project the following steps must be followed:
- In your Merge Module project go to Custom Actions Page.
- Select your defined custom action.
- From the context menu select the "Configurable" option and "Target" or "Source" depending on what type of parameter you want to set.
- The Edit Substitution Parameter dialog will appear where you can declare your parameters. For more info on how to create a Merge Module with configurable parameters for the attached Custom Actions, please see the tutorial: Create MSM packages with configurable parameters for custom actions