If you have one field, Field1 and you want to put the same value in every record in the table where Field1 is empty:
Perform a find, using a lone = in Field1 to find all records where Field1 is empty.
Use Replace Field Contents (this is NOT find/replace) from the records menu to put your specified value in every record in your found set.
This can be done manually or you can write a script.
If you have several fields, Field1, field2.... in the same records where you need to do this, repeat this process for each such field.
Note: since Replace Field Contents can modify literally 1000's of records in a table and cannot be undone, make a back up copy of your file befor trying this for the first time so that you can start over if you don't get the results you expected.