You can do that from your admin area - wrapper... if you do it that way you need to click on wrapper and insert your code for the header and then save it. After you save it go to 'manage forums' and edit your forum and change the part that says ' Forum wrapper' you will see the wrapper you just made. If you want that header to appear on all forums you will need to edit each forum and change that setting to the wrapper you made.

When I done it that way the header did not come up on the main forum so I made these changes to 'Skins/Classic/MasterTemplate/MasterTemplate.ascx'

Open that up and you will see whats below

<InstantASP:Form id="Form" method="post" runat="server">

Put your code for the Header here

<InstantForum:Content id="PageContent" runat="server">

Put your code for the Footer here

</InstantForum:Content>

<!-- ViewstateHere -->

If you alter the mastertemplate.ascx then all pages on your forum will have the header on it and remember to remove any custom wrapper or the top of the page will have 2 headers on it.