They are both proper German and have a similar, but not identical meaning.
Diese Filme werden noch nicht im Ausland gezeigt.
These movies are not yet beeing shown abroad.
If these movies are currently shown inland and one might expect that they are also already shown abroad.
Diese Filme wurden noch nicht im Ausland gezeigt.
These movies have not yet been shown abroad.
If these movies are a bit older (months or years) and one might expect that they already have been shown abroad in the past (and probably are not shown any longer).