When creating partitions or selecting existing partitions in the manual partitioning tool, the mount point box does not accept keyboard input. The dropdown still works, so the default values (/, /home, etc.) can still be selected, but a new one cannot be specified. The partition size spinbox still works correctly, so keyboard input is definitely working.
This is ubuntu 11.04 bug #769043
From "Try Ubuntu" run the installer, alt+f2 run gedit , type your custom mount point , select and copy the text to paste in the installer.
From "Install Ubuntu" you can't alt+f2 or run gedit so, go to system settings type your custom mount point in the filter box, select and copy the text to paste in the installer.
copy and paste inside the text box