Open iTunes and select "File." Select "Add File to Library" and find the .RMX file you are wanting to convert. Once found, select "Open" to add it to the iTunes library.
Click "Edit" and select "Preferences." Click the button that says "Import Settings" and click the drop down menu under the "Import Using" section. Select "MP3 Encoder" and set the quality of the future MP3 settings to "Higher Quality (192 kbps)." Click "OK" to finalize your settings.
Click on the .RMX file in your iTunes library. Click on "Advanced" and select "Create MP3 version." A MP3 version of your file will then be created and automatically placed into your iTunes library.
Right-click on the newly created MP3 file and select "Get Info." On the window that appears, you will automatically see the "Summary" tab of the window. Under the "Where" heading on the bottom of the window, you will see the path in which the file is located. Make a note of this path so you will know where to find the MP3 file in the future.