For example your Blog Header Page Element widget code looks like this :

<b:widget id='Header1' locked='true' title='MY BLOG (Header)' type='Header'/>

You can see this by login at Blogger.com and click on Layout link on Dashboard. Then click Edit Html subtab of Template tab. Scroll down in Template code box to see above code. This code is wrapped in the Header section code like this :

<b:section class='header' id='header' maxwidgets='1' showaddelement='no'>
<b:widget id='Header1' locked='true' title='MY BLOG (Header)' type='Header'/>
</b:section>

As you can see in the first line the number of maximum widgets in the header section is set to one. This makes it impossible for you to add new widgets in the header. Also Showaddelement is set to No.

To add more page elements in header change this code to :

<b:section class='header' id='header' maxwidgets='3' showaddelement='yes'>
<b:widget id='Header1' locked='true' title='MY BLOG (Header)' type='Header'/>
</b:section>

Set the maxwidgets to the number of widgets you want to put in the header section plus one. This makes it easier to rearrange the widget order in the header. Also change showaddelement to Yes. This will add a "Add Page Element' link to the header on the Layouts page.

You can do the same with the Blog Posts section code :

<b:section class='main' id='main' showaddelement='no'>
<b:widget id='Blog1' locked='true' title='Blog Posts' type='Blog'/>
</b:section>

Add a maxwidgets=x code and change showaddelement to yes. Save Template and click on Page Elements subtab of Template tab. You can now add any page Elements to the header and Blog posts. Do the same for the footer.