Please setup TYPO3 according this
The howto is written for TYPO3 version 4.2.0, but you can change the version number into 4.2.1. and it will work.
Within the webfolder web/ rename _.htaccess (which comes with TYPO3) to .htaccess
Or make your own .htaccess file with the directives mentioned by you.
Important is that .htaccess files arre allowed. If not it can be done by adding the following directives in the Apache directives field for tthe site:
Where web# is the webnumber as defined by ISPConfig.