Validation code for zip codes?

[Miss Swan]

Hi
I use virtumart shipping and have experienced some problems with zip codes.
User can only write letters and numbers, no space or special signs.
I was told it should be done in validation code, I can write a note on the form to users telling them not to enter any special characters in the zipcode, this can be done using shopper fields at the administrator.
My problem is I don't understand how?
I go to administrator/ shopper fields/ zip and try to write something in the description field, but apparently that's not the way to go.
Can someone please tell me what to do, where and how please?