You need to insert \limits
after \iiiint
:
\documentclass{article}
\usepackage{amsmath} % for \iiint macro
\begin{document}
\begin{equation}
m = \iiint\limits_E \rho \, \mathrm{d}V
\end{equation}
\end{document}
Please be aware that the default behaviour in displaymath mode is for the limits to appear above and below the symbol:
\[
\prod_{i = 1}^{n} a_{i} \quad \sum_{i = 1}^{n} a_{i} \quad \lim_{n
\to \infty} n^{2}
\]

The only exception to this is integrals:
\[
\int_{0}^{\infty} x^{2} \, dx
\]

The default behaviour in inline math mode is for the limits to appear to the side, e.g.
$\prod_{i = 1}^{n} a_{i} \quad \sum_{i = 1}^{n} a_{i} \quad \lim_{n
\to \infty} n^{2}$

The reason for this is to avoid irregular spacing between lines and excessive white space.
Compare:

With:

However, to force the limits to appear above and below in inline math
mode you can use \limits
as suggested by Christian Hupfer.
$\prod\limits_{i = 1}^{n} a_{i} \quad \sum\limits_{i = 1}^{n} a_{i}
\quad \lim\limits_{n \to \infty} n^{2}$

And to force the limits to appear to the side in displaymath mode, you can use \nolimits
\[
\prod\nolimits_{i = 1}^{n} a_{i} \quad \sum\nolimits_{i = 1}^{n}
a_{i} \quad \lim\nolimits_{n \to \infty} n^{2}
\]

Best Answer
Use the
\displaylimits
command.See page 16 of the amsmath package documentation for details (although you don't actually need amsmath for this).