user notification of multi-select drop down list

On my registration form I have a few drop down list that are multi-select. How do I tell users that they need to hold the ctrl button down and select multiple selections? Is there some kind of wording I can user about the multi-select drop down button? Have you come across this before where normal people that don't know much about computers need to select multiple options but don't know how to select them?

At this point how a multiselect works is basically common knowledge so it's not really considered anymore in designs. You could add it to the description or use a multicheckbox instead. In backend we've used "CTR/CMD-Click: Multiple" to explain multiple selection sometimes, but it's probably still not very clear if someone is very new to computers in general.