iNewsletter doesn't natively support tables, however there is a workaround you can use to insert tables.
Create the table in another program such as Google Docs or Microsoft Word and then take a screenshot of the table using a program such as the Snipping Tool (available in Microsoft Windows).
Once you have a screenshot of the table, you can insert it as a gallery. See here for more info on inserting a gallery.
We are constantly working on improving iNewsletter, and adding table support is something on our development list.