± Symbol in text

Similar to Christian's answer, but siunitx will deal with the correct typesetting, be it text mode or math mode.

\documentclass{article}

\usepackage[
  separate-uncertainty = true,
  multi-part-units = repeat
]{siunitx}

\begin{document}
Math mode: $\SI{50 \pm 2}{\percent}$

Text mode: \SI{50 \pm 2}{\percent}
\end{document}

\pm requires the mathmode, being switched into with $...$. For displaying % use \SI{number}{\percent} rather for better spacing and correct usage of fonts.

For text mode, there's \textpm too, this requires textcomp package:

\documentclass{article}

\usepackage{textcomp}
\usepackage{siunitx}

\begin{document}
$\SI{50}{\percent} \pm \SI{2}{\percent}$

Textmode: \SI{50}{\percent} \textpm\ \SI{2}{\percent}
\end{document}