Why doesn't this work? I think I've got all the packeges I need, and I've copied it from http://tug.org/pracjourn/2008-3/morales/morales.pdf.

\documentclass[landscape,a0paper,final]{a0poster}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{fullpage}
\usepackage[czech]{babel}
\usepackage{graphicx}
\usepackage{amsthm}

\begin{document}
\begin{minipage}[h]{0.5\linewidth}
{\bf Fermat’s last theorem}
If an integer $n$ is greater than $2$, then the equation
$$a^n+b^n=c^n$$
has no solutions in non-zero integers $a$, $b$, and $c$.
\end{minipage}}
\end{document}


Run it with xelatex:

\documentclass[landscape,a0paper,final]{a0poster}
\usepackage{fontspec}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{fullpage}
\usepackage[czech]{babel}
\usepackage{graphicx}
\usepackage{amsthm}

\begin{document}
If an integer $n$ is greater than $2$, then the equation
$$a^n+b^n=c^n$$
has no solutions in non-zero integers $a$, $b$, and $c$.