Yes, you can use LabVIEW 6.1 with cFP 1804. According to: What Version of the FieldPoint Driver Added Support for the cFP-180x? Support for the cFP-180x was added in FieldPoint 4.1.2. You must have version 4.1.2 or newer in order to see the network module in MAX and use it in LabVIEW. In your case, it cannot be a newer version because the newer versions don't have support for LabVIEW 6.1. Check which version of the FieldPoint software is included in the CD that came with your cFP 1804. In case it is 4.1.2 then you are all set; in case it is 5.0 or a newer one you will have to download version you need. You can find drivers at ni.com --> Product Support --> Drivers and Updates. For Product Line select Distributed I/O FieldPoint. For Software select NI-FieldPoint. For version, you will see v4.1 and then it skips to v5.0. Try version 4.1.
If it doesn't work. You will have to contact an Application Engineer in order to get a service number and then we will provide version 4.1.2. Dial the free telephone number according to your country or region. You can find the number at ni.com. The number for Mexico is 01 800 010 0793.