How to insert plain text?

Have you tried the verbatim environment?

\begin{verbatim}
  Your text here.
\end{verbatim}

Without knowing what your “weird symbols” are, it’s difficult to suggest a solution to your problem.

Update: In order to embed a verbatim environment in a table cell, you need to change the column to a paragraph column (using either p, m, or b as the column specifier); e.g.

\begin{tabular}{|p{10cm}|}
  \begin{verbatim}
    Your text here.
  \end{verbatim}
\end{tabular}

See Wikibooks for more information.