If you've already built against the libraries be sure to recompile all classes and PRG files manually by using something like the following from the VFP IDE command line:
COMPILE CLASS classes\*.vcx ERASE FILE classes\*.fxp COMPILE classes\*.prg
to ensure all files are up to date when you run the code. If you've added the libraries to a project make sure that you also recompile your project explicitly with the RECOMPILE flag to force updating of all files.