If you didn’t change it it’ll either be password or the last 8 of the serial number. 

If you didn’t change it, you can factory reset it to get back in

I have been using admin for user name it that correct or is it something else 

It won’t work with admin

did you try factory resetting? Maybe it was changed

In general, when sb6183 is directly connected to a computer at the time of login on 192.168.100.1 (web manager ) it doesn't ask for any username or password.

 If in case it asks for the user name is admin (always no change on it ) and the password is password all in lowercase

Also, follow the below steps for a Factory Reset (If required) :

http://arris.force.com/consumers/articles/General_FAQs/SB6183-Factory-Reset