You cannot use the DNS snap-in to perform the subject task.
Use the Dnscmd.exe command-line tool.
To create a wildcard character record:
1. Open a CMD.EXE window.
2. Type the following command and press Enter:
dnscmd <DNSServerName> /recordadd <ZoneName> * A <IPAddress>
where <DNSServerName> is the NetBIOS name of the DNS server, <ZoneName> is the zone name where the record will be created, and <IPAddress> is address that the wildcard character record points to when the DNS server reads the record.