Select the field in layout mode, then choose text format under teh format menu. Select paragraph, then center.
Thanks a lot, but in a calculated field? (this is because I talk about the textstyleadd function.)
Thanks in advance
If you want the entire field centered, then follow mr_vodka's advice. Otherwise, you'll have to split the calculation up into multiple fields, and apply the formatting to each field individually. There is no Center/Left/Right option for TextStyleAdd.