Without knowing more about the error or about your version of Dreamweaver, operating system and other installed extensions it's hard to say what the issue may be.
However, these are basic steps you can take to try to resolve it until we know more or someone from WA gets involved.
If you have Cookies Toolkit installed, remove it. Fully remove all versions of Security Assist. Then clear your WinFileCache.dat file. This may be in a different location than this depending on your OS. Mine is located by typing in %appdata% in a windows browser and then finding this path Adobe/Dreamweaver CS4/en_US/Configuration.
The WinFileCache.dat file will have other chars appended to it but it will be the only one there with WinFileCache. Delete it. Then try installing Security Assist again and see if that helps.