So changing "jQuery()" to "$()" in Mantra will not resolve this issue.
You need to figure out if you're using the correct WordPress/Mantra versions combination (As of version 1.8, Mantra requires at least WP 3.3).
If this is correct, make sure the image links you set for the slider are correct (try to access them in a separate tab directly and see if they work and are publicly accessible).
If this is not the case, a plugin may be interfering with WordPress' bundled jQuery. Try disabling all plugins and see if the issue goes away.
If this isn't the case either (or you have no plugins), then it could be indeed an issue with the theme, and you should let us know about it :)