You need to add the test cases to your test plan.
You will see the option under Test Plan contents, select your test cases and add them.
Then you will see them under the execute page.
I have another question.
How does testlink deal with compatability testing, meaning the ability to run the same test cases on multiple platform?
Do you recommend creating a keyword for each platform? or maybe duplicating the test cases?
I tried to use keywords for compatability/platform testing and i'm not sure that it works well for me.
The problem is that if, for example i create 2 keywords (for 2 platforms), once i executed the tests having filtered by the first keyword, i no longer can execute the cases again.