Assigning CSS ID Selectors to the Table
Merge the Header and Footer Cells:
Click inside the first table cell and drag to the right to select both cells in the header row:

With both cells selected, click the Merge icon on the Properties inspector to merge both cells.

Click inside the first table cell of the bottom row and drag to the right to select both cells in the footer row and merge them, too. Your table should now look like this:

Assigning IDs to the Table Cells:
Now that your selectors are written you will assign them to your table cells.
Add Header ID:
Insert your cursor in the header row
Right-Click its <td> tag on Dreamweaver's Tag Selector bar (along the bottom left edge of the Dreamweaver design window)

Choose Set ID--> header
Add Footer ID:
Insert your cursor in the footer row
Right - Click its <td> tag on Dreamweaver's Tag Selector bar (along the bottom left edge of the Dreamweaver design window)
Choose Set ID--> footer
Add Sidebar ID:
Insert your cursor in the left cell of the middle row
Click its <td> tag on Dreamweaver's Tag Selector bar (along the bottom left edge of the Dreamweaver design window)
Choose Set ID--> sidebar
Add Main Content ID:
Insert your cursor in the right cell of the middle row
Right - Click its <td> tag on Dreamweaver's Tag Selector bar (along the bottom left edge of the Dreamweaver design window)
Choose Set ID--> maincontent
Your table should now look like this:
