These instructions should work on all themes will little or no changes.

For the Corporate plan:
1) Login to the phpbb acp (http://yourdomain.com/phpBB3folder/adm/)
2) Click “styles”
3) Click “Templates”
4) Click the edit next to “prosilver”
5) In the drop down box find overall_header.php
6) At the end of that file add [code] <div align="center">CORPORATE/ADVANCED AD CODE HERE</div>
7) Save
Repeat for all other templates listed
You might have to click refresh next to the theme listed for the change to show

For the Advanced plan:
1) Login to the phpbb acp (http://yourdomain.com/phpBB3folder/adm/)
2) Click “styles”
3) Click “Templates”
4) Click the edit next to “prosilver”
5) In the drop down box find overall_footer.php
6) Find
Code:
 <!-- IF U_ACP --><br /><strong><a href="{U_ACP}">{L_ACP}</a></strong><!-- ENDIF -->
7) Hit enter and add the following after it
Code:
<br/> CORPORATE/ADVANCED AD CODE HERE
7) Save
Repeat for all other templates listed
You might have to click refresh next to the theme listed for the change to show

If you still can't insert the ads, please post in the "Free Hosting" board.