Custom taxonomy; link with two content types
Drupal 7

Go to Structure > Content types, select "Manage fields" for the post content type. Either add the existing taxonomy field to it or add a new one. For a new one, choose which vocabulary should be used for this field.

Drupal 6

Go to Admin > Content > Taxonomy, edit your vocabulary. Then under content types, select the content types for which this vocabulary should be displayed.