The problem with spell-check on worship software is the number of intentionally misspelled words it would find. Think of all the ones in scripture references that aren't part of modern spelling, then there's lyric contractions in songs (must have had 30 of those in our traditional hymn sing this week alone). The work-to-results ratio just doesn't add up...
The only way I have been able to get the spell check to work is slide by slide. Luckily it is in the right click menu for MS 4.5 while you are still editing it.