Unfortunately, curling is something that is common with quite a few Tunisian Crochet stitches. You can reduce the curl by using a hook size that is 2mm bigger than the recommended hook size for the yarn you are using. Blocking also works to reduce the curl afterwards. Another way that helps is by joining the Tunisian Crochet fabric to another piece, like in my Tunis Market Bag. Or you could add a border of regular single crochet, which also helps. Finally remember to pull the stitches up to the full height of the row you are working on.