How do I install a LaTeX .sty file on OSX?

I have a LaTeX project set up:

tex/
  - documents/
      - some_file.tex
  - support/
      - todonotes.sty

where some_file.tex uses todonotes:

\usepackage[colorinlistoftodos,textwidth=0.9\marginparwidth]{todonotes}

But I get "LaTeX Error: File `todonotes.sty' not found" when I try to build the PDF in TextMate. How do I tell LaTex or TextMate about my .sty file?

Later

To be specific, I'm using tetex from MacPorts, though given the answers so far, I might try another distribution.