After selecting Add New Source from either method described in the Datasets - Manage Sources knowledge article, a new source screen will be displayed, select 'Xero' from this list of available connections.
A login screen to Xero will appear.
After logging into the software, you will be asked to allow Active Ledger & Reporting (formerly Reportance) to have access to Xero. Select Allow Access to continue, on the next screen select Connect.
This will redirect you back to Active Ledger & Reporting (formerly Reportance) where a list of your available Xero files will be displayed. Select the relevant client from the list. 
Select Connect to establish a connection to the client file.
By default the name of the source will be the Xero file name, this can be changed if necessary. Finally, select Add source to finalise the connection.
Return to Datasets - Manage Sources