MacDNS starts the serial number at 1 and increments it by 1 each time the zone file is saved. ("DNS and BIND," published by O'Reilly and Associates, describes this method as one of the good ways to manage serial numbers.) The YYMMDDnn format is not part of the DNS specification.
However, you can satisfy your provider's requirements simply by using a text editor to open and edit your zone file. The serial number is stored as part of the SOA (start of authority) record at the beginning of the file; it's the first number following the administrator's e-mail address.
Example: If today is February 5, 1997, and it's the first change to this file today, you would change the number to 97020501. After you make this change, just re-open the zone file in MacDNS and you're set.