I can confirm wav impulse files do not work in Altiverb 5 on a mac. sd2 is needed. Not on either one of my Mac's any way. 3 of them. I don't know about Altiverb 6.
Search for Kazrog's impulses, they're pretty good and natively in .sd2 format, so that'll get you started (and eliminates the potential for some degradation in the conversion process)