You have a few changes to make. First, go back to your newly modified Multiselect. The overview is that in the "Token Options" section, you need to tell it how to build the portion of the search string you need, then in the search you'll need to tell it to use that token.
In my sample case (tossed together from some firewall information), I used a multiselect drop down to pick my "OUT" interface. This is ONLY an example, give more specifics and we can probably help more, but hopefully this will be enough.
So, first, the search I need the dashboard panel to run should look something like the below. You may want to open the search for that panel in Search and figure out what you actually need here, or maybe you already know well enough.
index=myindex sourcetype=mysourcetype OUT=X OR OUT=Y OR OUT=Z | blah blah.
So for my Token Options, I have
Token : out_tok (just a naming convention I learned for tokens ages ago).
Token Value Prefix : OUT=
Delimiter : OR
(Note, there is a space both before and after that " OR ")
If you watch the little "preview" section below the delimiter as you do this, you'll see it show you an example like OUT=value1 OR OUT=value2 OR ... and since that's exactly what I want to see, we're good here.
A note: you probably won't have to change the Dynamics Options Search String here (farther down the multiselect's properties), but you MAY want to run it in Search (there's a link right below it) to see what it actually outputs.
Now, on to the changes to the dashboard panel.
Click the Edit button for the search (Magnifying glass) and "Edit Search". You need to now wedge your token (which again will end up like "OUT=X OR OUT=Y") into your search, preferably the base search but that may take fiddling to figure out where's best. So in my case, I had
index=myindex sourcetype=mysourcetype | eval source_isLocal=....<big long nasty thing here>
So I modified it to
index=myindex sourcetype=mysourcetype $out_tok$ | eval ....<big long nasty thing here>
After that I saved the dashboard, then REFRESHED THE PAGE WITH F5. This isn't ALWAYS necessary, but can often be.
Then my drop down showed the two interfaces, each of which I could select, and which then filtered my dashboard panel.
Here's the doc for multiple value selections, There are links in there for each option type and how to put things together. A little searching or digging will turn up a lot more, including several Splunk education courses that will help too.
