It seems there is a ton on confusion about how to programmatically use multi-value parameters.
Here's how to do it using the SOAP API for report execution:
Here's how to do it in subscriptions:
Here's how to do it using expressions inside a report:
Now, just for your inner geek, here's a treatment of how NULLs work in multi-value parameters (they don't, but this post includes some work arounds):
(Update 04/25) Look like I missed a popular post on how multi-value parameters and subreports:
And of course the real product documentation is here:
Take care and good luck,