Add t('String to be translated') to your database
Every single string passed to t($string) function will be added to the database, and you can find it in the path you montioned above "admin/config/regional/translate/translate"

clear the cache, and search it again with help of filters existing on the admin translate page.

[EDIT]

First of all a must have module in multilangual sites is the internationalization module, it's a powerfull and helpfull module...

But the most important thing in your case is that before you can see your string in the transltion page:


you must enable(switch to) the desired language
visit the page that contain that needle string
visit again the translation interface.


That's it you will find your string ;)