The Drupal 7 translation system including the internationalization package is a heavy beast and while it mostly gets the job done, it is all but intuitive in use.
For high volume translations it is recommended to use the translation template extractor and translate the strings externally. For small corrections however, it is often much more convenient to use the translation interface (admin/config/regional/translate/translate).
Now that you've created that shiny module/template and made it translatable by passing all strings through the t() function, you may be wondering why your newly created string is not showing up in the translate interface.
To save you some trouble, here is a short list of things to check:
1. Include the project version number within the module/theme .info file.
Without the project version information, the translate interface will not register new translatable strings in your module/theme. Make sure to add version = 7.x-1.0 into the module/template .info file.
2. Run the string through the function in a non-standard language mode.
In order for the translation system to add your translatable string, the t() function must run at least once in a non-standard language. To achieve that, you will need to switch the language of the site to one you are going to translate into and then visit the page that displays the string.
3. Mind the case sensitive search filter.
Be accurate when using the translate interface filter - it does not forgive.
4. Flush caches if necessary.
Usually this is not needed, but maybe the new translatable string gets called on on a cached page or a cached view. With many layers of caching, better make sure and rule out all possible errors.
Let me know if anything is missing in this list.