Thanks for your answer. There are more than 100 users. It's tough to guess how many knows they can hold ctrl key and select a value. LED Check-boxes like Styling would do the job. It is only available in List boxes though. Do you have any other work around for this?
Understood but it has to be part of the user training. The other way you can try is as below:
In properties box you have a tab called "Caption" and almost to the bottom of that screen you have field called "Help Text". Here you can type the message. I typed as "Hold Ctrl key to select multiple values". Click ok and you will see a " ?" on the list box bar. When user hover over on the question mark or when they click on it, it will show the message.