I read a solution to adding custom tab in magento on magenticians blog. They have great tutorial their. You should search on this on their website.

the problem might be here

Code:
<?xml version="1.0" encoding="UTF-8"?> 
<config> 
<tabs> 
<inchoo translate="label" module="mymodule"> 
<label>Inchoo Extensions</label> 
<sort_order>100</sort_order>
...