This has been discussed many times before in the Band In A Box (Windows) forum.
If you want 1/4 note triplets to play properly, the best way that I have found to do it is via the Piano Roll editor. Note entry via this window has "Snap" and "Dur(ation)" controls that, when working in tandem, give perfect triplets. (Whether these notes display properly on the lead sheet, though, is another thing and depends on other settings. Even if they don't look correct on screen in Editable Notation mode, they will play correctly when set in the Piano Roll mode.)