Currently uploading a CSV file for import saves the file (temporarily) to the /plugins/ directory, which requires that you change folder permissions to allow it to do so.
This seems a little weird -- why not use the actual WordPress upload directory? Then you don't need to worry about permissions, as this is the appropriate folder for uploads.
I suggest changing anywhere in the file import-export.php it uses $file_location = ... to the following:
$file_location = $this->get_import_path($blog_id . '.csv');
using
function get_import_path($file) {
$upload_dir = wp_upload_dir();
// fallback to original path...?
if( isset($upload_dir['error']) && !empty($upload_dir['error']) ) return WP_PLUGIN_DIR . '/sm-temp-csv-' . $file;
return $upload_dir['path'] . '\\' . $file;
}