There appears to be a bug in magento 1.5x where by manually creating an order and trying to add a product does not work.  It appears to be an issue with ext-tree.js which is called unnecessarily on this page.

To fix edit app/design/adminhtml/default/default/layout/sales.xml

 

and add


<action method="removeItem"><type>js</type><name>extjs/ext-tree.js</name></action>

like so:


<adminhtml_sales_order_create_index>
<reference name="left">
<action method="setIsCollapsed"><value>true</value></action>
</reference>
<reference name="head">
<action method="removeItem"><type>js</type><name>extjs/ext-tree.js</name></action>
<action method="addJs"><file>mage/adminhtml/sales.js</file></action>