As far as the content or even appearance is concerned, an individual cell within a Table in PowerPoint acts just like any other text box. Therefore, various text alignment options used for text boxes work with Table cells too apart for a few exceptions. Once a cell is selected, the Text alignment options for that cell can be found within the Table Tools Layout contextual tab of the Ribbon — you can find them contained within the Alignment group.
Learn about the options for text alignment within Table cells in PowerPoint 2013 for Windows.

