THe process for getting data out of Btrieve files depends on what you've actually got. For example, if you are using a recent version of PSQL (v2000 or later) and have DDFs for your data (DAT) files, then all you need to do is create an ODBC DSN. Once you've got an ODBC DSN, you can either write a program to access the data and extract it or use a third party program to export it to various formats (like my DataXtraction which can export to CSV file format).
If you don't have DDFs, you should first contact the vendor of the application to find out if DDFs already exist. If they do not, then you will need to create them using DDF Builder or CREATE TABLE SQL scripts. You will need to know the exact record structure and data types to get the best DDFs. The other option is to use the Btrieve API which does not need DDFs but still needs the record structure because there is no field information stored in the DAT files.