Sorry to reply in English, my Brasilian Portuguese is not good enough…
Just go to Project->Properties, then Amateras and disable the JSP validation, this will be solved. This HTML plugin is too strict with this kind of syntax.
Otherwise, if you want to keep the control on the code validity, go to Window->Preferences, then to Amateras->TLD and add your taglib to the list…
Até logo,
Charles de Streel