I'm fairly new to Visualforce.  I'm working with custom objects in a managed package, attempting to create an email template with a table of related detail rows.  My visualforce email template is set to use the custom object "Sales Order":

<messaging:emailTemplate recipientType="Contact"
    relatedToType="PBSI__PBSI_Sales_Order__c"
    subject="Order and Shipping Confirmation for PO {!relatedTo.PBSI__Customer_Purchase_Order__c}">    
<messaging:htmlEmailBody > 

And I'm trying to access a related list custom object (Shipped Sales Order Lines) that has a lookup to the Sales Order but not a master/detail relationship.  
<apex:repeat var="ssoLine" value="{!relatedTo.PBSI__Shipped_Sales_Order_Lines__r}">

I'm getting an error :   [Error] Error: Invalid field PBSI__Shipped_Sales_Order_Lines__r for SObject PBSI__PBSI_Sales_Order__c.

I've tried using {!relatedTo.PBSI__Shipped_Sales_Order_Lines__r} as well as {!relatedTo.PBSI__Shipped_Sales_Order_Line__r}, but no luck.  Will this work with a lookup field relationship, or does it need to be master-detail?  How can I find the correct API Name if it is possible with only a Lookup?  If it's not possible, do I need to go the custom component/controller route, or are there other methods to access the related data?

Thanks in advance!

Shipped Sales Order Line info:
Singular Label: Packed Sales Order Line 
Plural Label: Shipped Sales Order Lines 
Object Name: Shipped_Sales_Order_Line 
API Name: PBSI__Shipped_Sales_Order_Line__c

Lookup field on Shipped Sales Order Line
Field Label  API Name                     Installed Package   Data Type
Sales Order PBSI__Sales_Order__c Ascent4Salesforce Lookup(Sales Order)

As a side note, I am able to access related Sales Order Lines, which do have a master/detail relationship to the Sales Order:
<table>
    <apex:repeat var="soLine" value="{!relatedTo.PBSI__Sales_Order_Lines__r}">
    <tr>
        <td width="20%">{!soLine.PBSI__Item__r.Name}</td>
        <td align="center">{!ROUND(soLine.PBSI__Quantity_Needed__c,0)}</td>
    </tr>
    </apex:repeat>   
</table>