Step 1. Go to your dash board and click on Editor as shown below
Step 2. Click on the Header template on the right
Step 3 : Locate the following lines of code near the end of the header file
and place your cursor in between these lines as in picture above (Click on picture to expand).
Step 4: paste your adsense code for the 728×90 Leaderboard :
Do not forget to sandwitch your code with the <div> </div>
You can try pasting the code between the lines below for testing purposes,
=============================================
<div align=”center”>
<script type=”text/javascript”><!–
google_ad_client = “xxxxxxxxxxxxx″;
/* 728x90Leaderboard */
google_ad_slot = “xxxxxxxxxxx″;
google_ad_width = 728;
google_ad_height = 90;
//–>
</script>
<script type=”text/javascript”
src=”http://pagead2.googlesyndication.com/pagead/show_ads.js“>
</script>
</div>
=============================================
when done with that , Click 
,
Then load your page to see if it worked.
If it works well you’ll see the 728×90 Leaderboard advert centered nicely just below your header image.
Do not forget to replace the ” xxxxxxxx” above in the [ 1-google_ad_client and 2-google_ad_slot lines] with the your own specific data.
