The ZX81 Tape converter operates against sound samples. You can either play an existing sound sample that contains a ZX81 program, or you can load a .P or .TZX file using the File -> Open menu option in the Main Tape Converter Window, which will automatically generate a temporary file containing the corresponding sample.
You can then save this sample file to be played later using the File -> Save As menu option.
You can use the Play -> Play menu option to play the sample directly.
Note that the temporary file is deleted when you quit the program).