Hi Adnan
Did you install DTIPrep through 3D Slicer or the 1.2.7 version on the download page here at NITRC? Calling it through Slicer may be a bit of an issue (I don't think the Slicer Mac version supports the --launch command line option).

If you downloaded from NITRC, then the setup should be hopefully straightforward, probably not unlike what you did for FSL or FreeSurfer, though I am no Mac expert.

If the issue is libraries, then "otool" may come in handy to pinpoint which libraries are not being found.

Martin