Assuming that this is not a permission issue and you have clear cache and re-login.
In config.xml
<?xml version="1.0"?>
<config>
....
<adminhtml> <---
<menu>
<customer>
<children>
<testimonials>
<title>Manage Testimonials</title>
<sort_order>0</sort_order>
<action>adminhtml/adminhtml_testimonials</action>
</testimonials>
</children>
</customer>
</menu>
</adminhtml> <---
....
or
In adminhtml.xml
<?xml version="1.0"?>
<config>
...
<menu>
<customer>
<children>
<testimonials>
<title>Manage Testimonials</title>
<sort_order>0</sort_order>
<action>adminhtml/adminhtml_testimonials</action>
</testimonials>
</children>
</customer>
</menu>
...