Ive not used the FS5, but based on prior experience the camera must be specifically set to operate in NTSC/60 hertz mode before you will be able to access 23.98p.
Reading from the FS5 user manual, pages 137 through 140, you need to select the System menu, then the item called "60i/50i SEL" and set it to "60i". Once you do this, the cameras recording and playback options will all be NTSC modes. Don't forget to switch it back to "50i" for your regular PAL clients when done.
Please further note per the manual, you cannot mix 60i and 50i footage on the same SD card - make sure you have a fresh formatted card for recording 60i.
And yes, "60i" is a misnomer - you'll actually be recording 23.98p. That's just what they're calling the 60 hertz NTSC format mode.