it's a command (that I typo'd and didn't catch oops), you have to (normally) be root or sudo it, but it should work in single user mode. Substitute <username> for the username you wish to make an admin. You could also probably enable the root account, log on as root and use the gui to promote your user to admin.
Like such.
mike-macbook-pro:~ mike$ id foo
uid=503(foo) gid=20(staff) groups=20(staff),98(_lpadmin),102(com.apple.sharepoint.group.2),103(com.apple.sharepoint.group.3),101(com.apple.sharepoint.group.1)
mike-macbook-pro:~ mike$ sudo dscl . -append /Groups/admin GroupMembership foo
mike-macbook-pro:~ mike$ id foo
uid=503(foo) gid=20(staff) groups=20(staff),98(_lpadmin),102(com.apple.sharepoint.group.2),103(com.apple.sharepoint.group.3),80(admin),101(com.apple.sharepoint.group.1)
mike-macbook-pro:~ mike$