Why won't iphoto slide show let you use iTunes songs
The option is there, but it simply does not work. Lots of workarounds on the internet that also do not work. Very disappointed in Apple...again.
No need to be disappointed with Apple at all. It does allow you to use iTunes songs.
Now if you want help figuring it out remember that we can't see your machine, so you're going to actually have to describe the issue in enough detail for us to understand it clearly. Then also explain what workarounds from the internet you have tried - that's so we don't waste time making suggestions you've already tried. Be sure to include the version of iPhoto and the OS you are using.