It would be something like:
WHERE
DATEDEIFF (d,[dca3_AeX AC Identification].[Client Date], GETDATE())>30
The easiest way to add tables and fields is to, in SQL Server Management Studio, expand the Symantec DB and right click on Views > New View. Paste this part of the SQL into the third pane down:
SELECT
[vri2_Computer].[Guid] AS [_ItemGuid],
[vri2_Computer].[Name] AS [Workstation Name],
[dca3_AeX AC Identification].[Client Date] AS [Last Checked In],
[dca3_AeX AC Identification].[OS Name],
[dca3_AeX AC Identification].[System Type]
FROM
[vRM_Computer_Item] AS [vri2_Computer]
LEFT OUTER JOIN [Inv_AeX_AC_Identification] AS [dca3_AeX AC Identification]
ON ([vri2_Computer].[Guid] = [dca3_AeX AC Identification].[_ResourceGuid])
WHERE
(
(
(DATEDEIFF (d,[dca3_AeX AC Identification].[Client Date], GETDATE())>30)
)
Then you can select more fields in the top pane or add more tables or views. When you add more tables you need to create the joins by dragging and dropping fields from different tables to join them. Usually GUID fields.