If you are not running Mambo 4.6.1 then you should patch up to this version prior to applying this new patch.

Steps:
1) Take your site offline: Site->Global Configuration->Site Offline (YES).
2) This patch includes a complete update for the MOStlyCE editor . To make sure those update goes smoothly follow these steps:
* If you have made customizations to your exitising MOStlyCE install (ex) icon changes, enabling/disabling plugins, etc then first make a local backup copy of your MOStlyCE configuration file . You can find it at the following location mambots/editors/mostlyce/jscripts/tiny_mce/mostlyc e_config.php.
* Next remove these three items (they will be replaced by the patch and we don't want to leave old artifacts around):
a) mambots/editors/mostlyce (the entire directory)
b) mambots/editors/mostlyce.php
c) mambots/editors/mostlyce.xml
3) Extract the patch into your existing Mambo 4.6.1 installation and let the files overwrite the existing files with the same names. The patch mirrors the existing Mambo 4.6.1 file/folder structure so the files will go into the correct locations when extracted.
4) Run the following SQL statement via your normal database interface (ie) phpMyAdmin, MySQL commandline, etc. You will need to replace "#_" with whatever your table prefix is. By default the Mambo table prefix is set at "mos_" (Note: that is a single underscore).

5) Open you configuration.php file. Now verify and/or change the following variable so that they match what is shown below:
* $mosConfig_lang = 'english';
* $mosConfig_locale = 'en';

6) Bring your site back online: Site->Global Configuration->Site Offline (NO).
7) If you are using the MOStlyCE editor then you need to do the following to finish off the editor update:
* If you made a backup of your mostlyce_config.php file in step 2 you should now put that back in place (mambots/editors/mostlyce/jscripts/tiny_mce/mostly ce_config.php).
* Verify your mostlyce_config.php file is writeable (mambots/editors/mostlyce/jscripts/tiny_mce/mostly ce_config.php).
* Toggle the editor off and then back on to re-register it within your installation. You can do this via the first tab inside the global configuration interface.

Note: Failure to follow the steps above could cause the editor to load improperly and just display as a plain white textarea after the patch. If you experience any issues with the editor after following these steps please see the following thread on the forums: http://forum.mambo-foundation.org/showthread.php?t=3001

That's it. Patch Complete.