What you said worked, but I have another issue now. The image below was generated using MikTeX on Windows,

The same code, when run through MacTeX on OS X gives this, however:
(Here are the links to the images
http://img132.imageshack.us/img132/4190/picture1tgu.png and
http://img132.imageshack.us/img132/3316/picture2hoq.png in case they're not displaying right above.)
The actual code that's in the images above is
Code: Select all
\chapter{Semigroups \& Monoids}
One of the most basic algebraic structures we can place on a set is
that of a magma. A \definition{def:magma}{magma} is simply a set
along with a binary operation -- some way of
\marginpar{A binary operation on a set $S$ is a function $\cdot : S
\times S \to S$ where we usually write $x \cdot y$ for $\cdot(x, y)$.}
combining two elements in the set and
getting another element in the set.
Where I'm using the memoir document class and the following in my preamble:
Code: Select all
\setlength{\marginparwidth}{1.2in}
\definecolor{grey}{gray}{0.5}
\let\oldmarginpar\marginpar
\renewcommand\marginpar[1]{\-\oldmarginpar[\raggedleft\footnotesize \textcolor{grey}{#1}]%
{\raggedright\footnotesize \textcolor{grey}{#1}}}
\let\oldchapter\chapter
\renewcommand\chapter[1]{\chapterstyle{pedersen} \oldchapter{#1}}
Any ideas?