How to add a dkim entry with selector "mail"?

When we setup a server by default dkim entry comes with the selector- "default". But when I want to  use a separate mail server (in my case I'm using mail-in-a-box server) then I need to put another txt (dkim) record with  "mail._domainkey" in cwp. but when i am doing the entry it says
"Error
There is some error with the configuration, please check and update"
if I add directly from the file (/var/named/example.com.db) then bind server stops working.
says
" dns_rdata_fromtext: /var/named/example.com.db:26: syntax error "

Already tried to add another dkim key with the selector"mail"
but nothing is working.

Need help.

thanks

This is my settings.  You must restart bind after each modification. hope it helps