I was looking to pass in multiple parameters for a given parameter using SSRS data driven subscriptions.
It took some investigation and a lot of trial and error.
However, the following link pointed me in the right direction:
It turns out you have to set the parameter's "Available Values" to NON-QUERIED as well as your "Default Values = to NON-QUERIED. And be sure to enter some dummy default values or it won't take.
Once you get the report uploaded and setting the values, you can simply select multiple values from the drop down list under Specify a static value:
Hope that helps!