Per the announcement on the front page the Nucleus CMS project has been closed.
Nucleus CMS core: The core code, as it was last under development, is available at Sourceforge. Version 4.0 was in progress when development was last active. Note: development has not been active on this since 2013 and it does not include updates made to LMNucleus.
The Nucleus CMS core development team has released Nucleus CMS v3.65. This release includes a few minor bug fixes that are intended to make it compatible with PHP 5.4. It is recommended that only users who are having issues with PHP5.4 support upgrade at this time. There are some bugs with character encoding in the Admin area.The change log can be found here.
The Nucleus CMS core development team has released Nucleus CMS v3.64. This release includes a major security fix and a few minor bug fixes. All users who allow comments from non-members should upgrade immediately (fixed in 3.64). All users who allow visitors to create member accounts (register) should upgrade immediately (fixed in 3.63). It is recommended that all users upgrade
If you are running 3.32 or ealier version, if you allow comments from non-members, or if you allow visitors to create member accounts, you should upgrade immediately. But we recommend all users to update so that your blog software is up to date and will work with all plugins.