Mareco,
Lookout 3.8 build 7 was released 1998. In the meantime National Instruments realesed Lookout 4 and 5. The latest version of Lookout (as of 4 or 5) would support signed Modbus input registers.
I would highly recommend to an upgrade to Lookout 5 that comes with among of other features! www.ni.com/lookout
Hope this helps
Roland