I've learned the best way to work with this is to copy the code and start there, but them you can switch to the compose mode to finalize the table editing such as centering texts, bolding it, etc.
In fact, the easiest way to do this is to format one row and then copy the HTML code.
Basic Table Code:
<TABLE BORDER=2> <TR> <TD>HEADER</TD> <TD>Header </TD> </TR> <TR> <TD>entry</TD> <TD>???</TD> </TR> <TR> <TD>entry 1</TD> <TD>???</TD> </TR> <TR> <TD>entry 2</TD> <TD>???</TD> </TR> </TABLE>
This is the code for columns <TD>???</TD> so if you want to add more columns, just add this code where you want additional columns.
Remember, this is html coding so you have to be editing html.
Here're are the results:
A very useful site for generating table codes with colors, etc.: http://www.quackit.com/html/html_table_generator.cfm