In the Data View tab,
Select the foreign key, say to your vendor. Below that line Ctrl-H these are your options
Once you pick an option, select the key (in this case, they key of the vendors unique ID), then use the locate from and to to 'find' the related record.
Expression 32 in this case points to the APVendor variable
Look up Link Definition in help, it has some 'help' but also links to the Mastering XPA document which has more detail
Lots more to it but this should get you started