Make sure the Smarty-2.3.0/ directory exists under the root of the xcart/ install. Compare it with your .tgz file contents, you should be able to open it with winrar, or such.
The require_once(); in line 1084 of Smarty.class contains a require for the .class but doesn't find it, so make sure its intact and you shouldn't be having a problem.
Did you get any [ERRORS] upon install?
hth.
