What exactly do you want to achieve? It's not clear from your description...
Sorry about that I didn't proof read it.
I want it so that when you start typing in letters into the combobox it automatically starts filtering the dropdown with items which contain the letters, so for instance for finance. If I searched for fin it would show the finance as well as any other departments which have the letter combination of fin in them.
OK, that's more clear. I don't think it's possible to do it with just a drop-down menu, though. You can do it using a separate text field that acts as the filter.
This requires using a script, of course.
I've developed a product that allows you to easily set up such a filter field. If you're interested you can purchase it from here: Custom-made Adobe Scripts: Acrobat -- Create Live Filtered List
Note that dropdowns in XFA forms (LiveCycle) behave the way you want without any scripting. Dropdowns in Acrobat don't behave that way, but you can cycle through the items that begin with a particular character by repeatedly pressing that character on the keyboard. Not exactly what you're looking for, but it might be helpful.
I did see that feature, unfortunately I need it so that others are able to edit the form and keep all scripting/fields etc.. who do not have Livecycle installed. I supposed they will have to live with it for now.