Commands paths in TeXstudio on Mac OS X - with 'could not start the command' error

Paths for a MacTeX distribution

The following settings should work:

latex       "/Library/TeX/texbin/latex" -synctex=1 -interaction=nonstopmode %.tex    
dvips       "/Library/TeX/texbin/dvips" -o %.ps %.dvi
ps2pdf      "/usr/local/bin/ps2pdf" %.ps    
MakeIndex   "/Library/TeX/texbin/makeindex" %.idx    
BiBTeX      "/Library/TeX/texbin/bibtex" %.aux    
pdfLaTeX    "/Library/TeX/texbin/pdflatex" -synctex=1 -interaction=nonstopmode %.tex    
dvipdf      "/Library/TeX/texbin/dvipdfm" %.dvi    
dvipng      "/Library/TeX/texbin/dvipng" -T tight -D 120 %.dvi    
Metapost    "/Library/TeX/texbin/mpost" --interaction nonstopmode     
DVI viewer  open %.dvi    
PS viewer   open %.ps    
PDF viewer  open %.pdf    
Ghostscript /usr/local/bin/gs "?am.ps"    
Asymptote   "/Library/TeX/texbin/asy" %.asy

If you have updated to a recent MacTeX and need to change them all at the same time, you can quit out of the app and edit the texstudio.ini file which is stored in ~/.config/texstudio. This is a plain text file and you can then do a global replace of the old path with /Library/TeX/texbin.

Paths for other types of distributions

If you are using any other TeX distribution, for example MacPorts or Fink, you will need to change /usr/texbin/ to the relevant path in that particular distribution. In MacPorts it is /opt/local/bin/.

I would not recommend using such a distribution, however, as they are generally harder to keep updated.

Screenshot

Note that old paths in /usr are not valid anymore in Os X 10.11+. All paths starting with /usr/texbin/ should be changed to /Library/TeX/texbin/.