Hi Reju,
It looks like this is a somewhat out of date installation of yt, probably from around the fall of 2011. In order to get SlicePlot to work, you'll need to reinstall yt, along with all of its dependencies.
We're only about a week or so away from the release of yt 2.5, so it should be safe to install the latest development version. To do so, run the following commands:
$ bash install_script.sh
If you don't have wget, you can achieve the same result using curl:
$ bash install_script.sh
The install script will print some instructions, you should read them and follow any suggestions it has, which will vary depending on your operating system.
Once you've installed the new version, you should be able to update yt by using yt update:
$ yt update
And update all of the yt dependencies by running:
$ yt update --all
Please let us know if you have any difficulties installing the latest version.
Cheers,
Nathan