Read this first.
Click customize and paste this onto the Description box:
<form action=”/search” method=“get”>
<input type=“text” name=“q” value=”{SearchQuery}”/>
<input type=“submit” value=“Search”/>
</form>

Hmm whenever I try to type something in it seems to say Not Found

The URL you requested could not be found.

Oh my mistake! It seems to be working now! Thanks alot! :)

I believe some tumblr themes have search options than other themes. it depends which theme your using.