Did you have any older versions of Photoshop installed on your Mac?
First, reboot your computer. Then before you begin installation, navigate to the "Activity Monitor" on your Mac.(can be found under utilities).
Once you open Activity Monitor, Select "All processes" in the dropdown list at the top.
Look for Photoshopserver and dynamiclinkmanager in the list of processes.
Select them and click on "Quit".
Try reinstalling Photoshop.
Do let us know if you continue to face the same issue.
thanks a lot!
after rebooting the imac it wasn´t necessary to open the activity monitor.
the installation works now!
a 1000 thanks...
and to anyone who has the same problems than me:
Gautham K rules!!!!!