The Keyword Assignment page is the place where users can batch assign keywords to the existing Test Suite or Test Case
Keyword assignments you make to the specification will only effect test cases in your Test plans if and only if the test plan contains the latest version of the Test case. Otherwise if a test plan contains older versions of a test case, assignments you make now WILL NOT appear in the test plan.
TestLink uses this approach so that older versions of test cases in test plans are not effected by keyword assignments you make to the most recent version of the test case. If you want your test cases in your test plan to be updated, first verify they are up to date using the "Update Modified Test Cases" functionality BEFORE making keyword assignments.