unable to install on 3.5.1

That is not an error from the WP version of 3.5.1, but is a PHP version issue. The newest version of the plugin now requires PHP 5.3+. If you’re on 5.2, which it sounds like you are, you could ask your host to upgrade you, or if they cannot, you can download version 1.0 which is the last version to support PHP 5.2.

how do i get the 1.0 version?

To get previous versions of any plugin, go to the developers tab of the plugin page
https://www.ads-software.com/extend/plugins/widget-settings-importexport/developers/