Hello Lucy,
I was having the same issue you seem to be having. Check the Organization Service URL that you copied from your CRM Customizations Page, eg: 'https://<org>.crm.dynamic.com/XRMServices/2011/OrganizationData.svc'.
The link that was in my CRM left out the word 'Data' after Organization. I simply typed in 'Data' back in and it worked for me. Hope this helps.