Solution:
Wow.. just found the problem and answer to my question below. Had to do with Compatibility settings in IE 9. Once I added my reporting server url to my Compatibility View websites, everything started working properly. I'll leave this post up here on StackOverflow in case someone else runs into this problem.
Thanks anyway and I appreciate all the answers I've found on this site through the years!
Original post:
I've created several reports within SSRS 2008 R2 that use multi-valued parameters. They all work properly when running the report directly, but when I try to setup a report subscription (standard, not even data-driven), I cannot select any of the multi-value parameters. I've done extensive research that does not indicate that multi-value parameters are not supported within SSRS 2008 R2 subscriptions, so I'm wondering if anyone else has run into this problem and how they fixed it. (If anyone knows otherwise and that multi-value parameters are not supported within subscriptions, please say so!)
One page I found online suggested a possible corrupt ReportingServices.js file. I have not modified my file at all, so I'm ruling that out (unless it's possible for the file to be changed some other way?).
I created a new, blank sample report with 2 simple parameters for testing and the same problem still exists. My SingleValueInteger1 has a combobox dropdown that works fine in the subscription, but my MultiValueInteger1 shows the dropdown arrow, hovering over it shows the text "Specify one or multiple default values.", but clicking on the arrow does not expand the field allowing me to select or even see any of the available parameter values. Both of these parameters have "Allow null value" unchecked, both are using the "Specify values" under the Available Values (not using "Get values from a query" to keep things simplified in this example), along with no Default Values.
I tried to include illustration pictures with this post, but it's my first post and I don't have "at least 10 reputation".
Thanks in advance for any suggestions or help. I've spent countless hours trying to figure this out!