this can happen if you have multiple versions of Dreamweaver installed, and you double click the mxp file to start the installation.
If the older extension manager is the default, you will get this error.
To prevent this, you will need to start the installation manually.
Open the CS5 extension manager, then click the install icon.
this will open a mac finder window, browse to the location where you saved the Pro maps MXP file to, and select it to start the install.