You need Modbus TCP client connectivity to be able to link to Prosoft MNET Servers for testing. Older evaluation or demo versions of Win-Tech's Modscan32 Modbus master simulator software did not allow use of TCP/IP Client connections unless you purchased the licensed version.
If you have an older version, first, completely uninstall it. Then, download the newest trial version of Modscan32 from the www.win-tech.com site, install, and try it.
And remember, the software is not freeware. If you continue to use it, you should purchase the license.