[OS X TeX] Table formatting

[Alireza Dehghani]

Hello, 
when i am using the table environment i use the following code to typeset a table:

\begin{table}
\caption {Mechanical properties of natural fibers (Saira Taj \emph{et al}. 2007)}
\label{Natural fibers}
\begin{tabular}{|m{2cm}| m{2cm}| m{2cm}| m{2cm}| m{2cm}| m{2cm}|}
\hline 
Fiber & Category & Specific gravity & Tensile strength MPa & Tensile modulus MPa & Specific modulus\\
\hline
Sisal & Leaf & 1.3 & 510 & 28 & 22\\
\hline
Jute & Bast & 1.3 & 393 & 55 & 38\\
\hline
Flax & Bast & 1.5 & 344 & 27 & 50\\
\hline
Pineapple & --- & 1.56 & 170 & 62 & 40\\
\hline
Glass fiber-E & --- & 2.5 & 3400 & 72 & 28\\
\hline
\end{tabular}
\end{table}
but the resulting table is unformatted and looks like the following image:
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why this happens? 
thank you...