Go to your blog backstage -> Appearance -> Widgets -> Drag a new Text to Sidebar -> copy the following code and paste to it:

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<form method="get" action="http://www.google.com/search">

<div style="border:0px solid black;padding:4px;width:20em;">
<table border="0" cellpadding="0">
<tr><td>
<input type="text" name="q" size="30"
maxlength="250" value="" />
<input type="submit" value="Google Search" /></td></tr>
<tr><td align="center" style="font-size:100%">
<input type="checkbox" name="sitesearch"
value="URL" checked /> Station Search<br />
</td></tr></table>
</div>

</form>

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just change URL as your site (e.g. 3w.xxxxx.xxx)
Some parameters like size, adjust yourself.
Check out what does it looks like, on the right side.