To convert MP3 files to WAV format (or to another of the supported formats) with FreeRIP, you must use View->Converter menu option to set FreeRIP in "Converter" mode.
Once FreeRIP is in Converter mode you can use File->Add files to convert... menu option to build the list of MP3 files to convert. Alternatively you can use Windows drag&drop: drag MP3 audio files from a folder and drop them onto FreeRIP window.
Once the source file list is complete, use Rip menu to convert them in the chosen output format. E.g. select Rip->Convert selected tracks to WAV to convert to WAV format, or select Rip->Convert selected tracks to FLAC to convert them to FLAC format .
If you have multiple format audio files to convert, you can add them all to FreeRIP Converter interface and bulk convert them.
For more details about FreeRIP MP3 Converter, look at Converting Files section of FreeRIP user manual.