Symptoms
Client Tools only installation requires database connection details.
Resolution
Referring to http://help.k2.com/onlinehelp/K2blackpearl/ICG/current/webframe.html_Client_Tools_Only_install.html if you are using the K2 Service account, you need to login with that account on the machine, choose 'Existing environment' and connect to the db.
If using a user account, you can choose the 'Manually configured environment' option, where you would simply need to enter the ports for connection to your K2 Server.