Even though the Samsung X830 does offer Java support, one can't download and install new Java applications on the phone - only the pre-installed Java applications will work on the phone. It is therefore (because MXit is a Java application) unfortunately not possible to make a version of MXit for the Samsung X830.
Samsung South Africa have updated their website and now no longer list Java support as one of the X830's features (previously incorrectly listed). This further confirms that the Samsung X830 doesn't offer Java support:
Jul 19, 2007 |
Samsung SGH-X830 Cellular Phone