Did you read the manual? You want to shoot 720p/24pN and then pick a record frame rate- 0-60fps. Shooting 720p/60 is not variable framerate it's 720p/60. If you want it to look slow mo now, use the Speed control in FCP and slow it down by a percentage. Make sure to click blend frames off.
You can convert 720p60 to 30fps or 24fps later. You can use the DVCPRO HD Frame Rate Converter (in the TOOLS menu), or modify the clip...called CONFORM...using Cinema Tools, to whatever speed you want. Duplicate the clip first if you want one at normal speed, because CONFORM modifies the clip permanently.