There was a previous post dealing with this. See:
The problem is with the write permisions for the search engine folder located at:
A quick way, but a way that makes that folder writable by all users, would be to change the permissions as follows:
chmod -R go+w searchplugins
You sould now be able to add search engines.
If you wanted to secure it after you have added you search engines you could change the permissions back with:
chmod -R go-w searchplugins