If you have already installed Notes Client on the Macintosh harddrive, and decide to upgrade to a newer release, the Lotus Notes Macintosh install defaults to a folder called "Lotus Notes", even with the Notes Preference file present on the machine. If you have Notes installed to a folder named something other than "Lotus Notes", the install doesn't find where the previous release is installed.
This issue has been reported to Lotus Software Quality Engineering. There are currently no plans to address this issue in Notes R5 Client.
Workaround: If you want to install Notes to the same folder you used for the previous Notes Client install, first locate it on the computer, note the folder it is located in, then start the installation. When prompted for the whereabouts of the Notes folder, type the directory/folder structure you noted.
This is documented in the Notes/Domino 6.0 Release Notes (on the Lotus Developer Domain web site) as follows:
Macintosh - Client Upgrade default folder
The Macintosh Installer defaults to the Lotus Notes folder, even if you have the Notes Client installed on your system in a different folder. When upgrading your Macintosh Notes Client, please be sure to change the installation folder, if needed, to assure the files are installed to the correct folder.
No Native Mac OS X Installer for Notes 6.0; OS 9 Classic Environment Required for Install
Document #: 1095738 (195838)
Lotus Notes Client