there are two ways.
First you have to make sure you are able to distribute the file. If it is legal, then you can include it as an additional resource with a script alias. then have a button called 'install codec'
When you add an additional resource, (Publication properties) it expects to see video or audio, but if you select all file types you can select an EXE.
Navigate to the file location and select the codec, then give it a script alias----this is what you will call it in the script function. Note he alias name should be enclosed in double quotes.
so
file name
C:\dir\subdir\thecodec.exe
Alias
"MyXVidCodec"
when you save the project, the file is then included in the resources.
next on your button, attach a script with
Code:
LaunchFile("MyXvidCodec")
that should do it. But you better make sure you can distribute the file.
You could also check for the existence of the file and only show the install button if it is not found (FileExists)
The next way is better in some ways. Rather than Launch File script action, you simply have a Launch URL which directs to the location of the codec on the web and you tell them to install it. This makes sure you always get the latest version and avoids any possible legal problems of distributing the file.