If you have FL Studio, open the FLP and choose "Export..." from the File menu. In the dialog that appears, choose among a variety of common audio formats supported by iTunes, including MP3 and 16- or 24-bit WAV. Windows users who do not own FL Studio can download and install a free demo from the Image-Line website. Though the save function will be disabled, the demo version of FL Studio does allow you to render FLPs to MP3 or WAV format.
If you run into problems using the demo version of FL Studio -- or if you are a Linux user and don't want to have to troubleshoot WINE -- consider downloading and installing Linux MultiMedia Studio. LMMS is a free and open source (FOSS) alternative to FL Studio with many of the same features and a similar user interface. Though FLP is a proprietary format, LMMS can read, write and render FLP files to audio. Open the FLP, choose "Export..." from the Project menu, then select "WAV-File" (for WAV format) in the dialog that appears.
As of November 2011, Image Line has yet to port FL Studio to OS X. Similarly, Linux MultiMedia Studio is only available for Windows and Linux. However, Mac users can install the Windows binary for LMMS by using Parallels, a commercial Windows emulator for OS X. Mac users can also use Oracle's freeware VirtualBox to run a multimedia Linux distribution that includes LMMS, like Dream Studio or KXStudio. If LMMS has not been installed by default, you can add it via Ubuntu Software Center or KDE Packager.
Once you manage to open your FLP file in FL Studio or LMMS, you may receive an error message stating that it includes references to instrument and effect plugins not present on your system. If this error occurs, make a note of the names of the missing plugins, then head to KVR Audio's Plug-in Database, a repository of reviews, specs and download links for commercial and freeware audio plugins in a variety of formats. If you can't find the plugins at KVR, turn to your search engine of choice.