The error indicates that you currently have a Lotus Notes Multi User client installed on the workstation. In order to install the Notes Admin Client/Developer client/Notes client, you will need to un-install the current notes client, and then re-install the new software package. When you install the new software (Notes Admin Client), the data directory will be placed in a different location by default, and you will need to move the files from your current Data Directory to the new location.