How can I translate those translatable texts defined in t() in contribute/custom modules? I wrote some custom modules, follow the Drupal standards, put all interface texts in t() function, make them translatable. Module enabled. 

I enabled Locale module, added new Language under admin/config/regional/language:


However, under admin/config/regional/translate/translate
I cannot see those text I defined t('xxx') in my custom modules. (cache cleared)

How can I make Drupal aware of them? Thanks.
You can use the Translation template extractor module to extract all the t() calls you've done.

Another option is to get the t()processed, by viewing the pages where they are defined and then translate them in the interface you mention. (New strings will only be added once they have appeared once).