How can I add abstract to table of contents

Within report, the abstract environment is set using within a titlepage, which defaults to putting the content on its own page. So you have to insert the ToC-writing piece using some patching (supported via etoolbox).

\documentclass{report}
\usepackage[paper=a6paper]{geometry}% Just for this example
\usepackage{lipsum,etoolbox}% http://ctan.org/pkg/{lipsum,etoolbox}

% \patchcmd{<cmd>}{<search>}{<replace>}{<success>}{<failure>}
\patchcmd{\abstract}{\titlepage}{\titlepage% Insert ToC-writing after starting a titlepage
  \addcontentsline{toc}{chapter}{Abstract}}{}{}
\setcounter{secnumdepth}{3}
\setcounter{tocdepth}{3}

\begin{document}

\title{My Title}

\author{My Name}

\maketitle

\tableofcontents

\begin{abstract}
\lipsum[1]
\end{abstract}

\chapter{A chapter}

\end{document}

I've written the ToC-entry as a chapter, just so the formatting seems consistent

\addcontentsline{toc}{chapter}{Abstract}

However, that can be changed to whatever you like.

---

Another way would be to write the ToC-entry within the abstract environment - this ensures that it falls on the same page as the Abstract:

\begin{abstract}
  \addcontentsline{toc}{chapter}{Abstract}
  ...
\end{abstract}

As abstract is neither a chapter nor a section so for abstract you should use

\begin{abstract}....\end{abstract}

as shown below:

\begin{abstract}
\addcontentsline{toc}{chapter}{abstract}

abstract content here
\end{abstract}

Line below is used to show abstract in list of contents:

\addcontentsline{toc}{chapter}{abstract}