Hi Frank, and welcome to the forum.
That’s an odd alert from Toast, but I can explain it.
PAL format is 25 fps. NTSC is 30 fps (technically 29.97). Both of these standards were for over-the-air broadcast before the digital TV era. PAL and NTSC have been replaced by digital broadcast formats around the world.
Digital video formats online can have a wide variety of frame rates. 24, 25 and 30 fps are most common. 25fps is the most recommended frame rate for general use. Soundslides 3 exports at 25fps. The original Soundslides converter, which predated the transition to digital broadcast, exported at 30fps only to maintain compatibility with the now obsolete NTSC.
My guess is that your version of Toast is reading the “25 fps” part of the file, identifying it as the PAL frame rate, then recommending a conversion to NTSC. Either way, pre-converting from 25 fps to NTSC is a fairly normal thing to do, and it should work without quality issues with audio slide show source material.
The last time I made a DVD, the frame rate pre-conversion was automatic and didn’t require any additional steps, but that might depend on the software you’re using (I was using iDVD).
Hope that helps, and that I didn’t go overboard providing too much background on old TV formats.