AssetAccountant will detect assets entered into Xero only AFTER you have setup the integration and they are added to an account with the account type of 'Fixed Asset'.
You can quickly check your chart of accounts to see which, if any, accounts are set up in this way in Xero:
To test this is working, try coding a fixed asset acquisition transaction to an account of this type in Xero, then have a look in AssetAccountant - you should then have the option to Add or Ignore any such transactions.
For existing assets in Xero, you will need to import them into AssetAccountant
You can easily export these from Xero and import into AssetAccountant.
Please see this Xero article for details: