It sounds like you have the right file, but the file is not a “program”. The file is just a list of settings that can be loaded into Audacity’s “Equalization” effect.
You need to
- open your Audacity project,
- select the track that you want to apply the roll-off effect to
- Open the Equalization effect (Effect menu)
- Click the “Save / Manage Curves” button.
- Import the “LF_rolloff_for_speech” XML file. See here for more details about this step: Audacity Manual
- Return back to the main Equalization effect interface and select the new “LF Rolloff for Speech” preset.
- Adjust the “Length of Filter” slider to maximum
- Apply the effect (press the OK button).