When i had to change the mapping in MSCRM 2011 Online I had a problem getting the mapping id because MSSQL is not reachable in ONLINE version.
The easiest way to get the mapping Guid was using the OData service.
1)* Go to the link SERVER URL/XRMServices/2011/OrganizationData.svc/EntityMapSet()
2) Find the Guid and saved it somewhere.
3) Go to any other relationship in QuoteDetail in your crm (If you can't see the url bar click ctrl+n)
4) Open the Mapping and get by using the developer tool it's url.
5) Replace the Guid from the last part with the mappingId
6) Open the new link in the browser
The link should look like
*It is also possible getting the Guid using ODataTool solution that is also very useful for other purposes.
Hope it helps..