i was trying to open terminal window from HMC its giving me the below message .
Unable to Switch Tasks
The task you want to switch to is an applet based task, which does not support 'Switch To'. Please use the window switching capabilities for local windows to 'Switch To' the window for this task.ACT01513
Maybe your HMC needs rebooted? You can open a terminal window through a SSH connection to your HMC instead until it is fixed by running "ssh hscroot@hmc_console", then run "vtmenu", select your managed system, then select your LPAR and you will have a console window open to it. I use this method instead of the Java based console through the HMC GUI because for some reason Java runs real slow on my workstation. If I open up two or more, they all get almost non-responsive.
hehe have you maybe pressed no where you should have pressed yes - in the newer hmc versions, the java security confirmation window changed - I keep being trapped by the same issue - only complete logout and login - and than confirming with the correct button - helps 
Regards
zxmaus
HOLY CRAP.....That was my issue...I was going through too fast on the Java message...I can't tell you how long I've been dealing with this "issue"...
Sometimes as techs we need to remember even WE have to read dialog boxes.
Thanks
We just ran into this same issue last week during some disaster recovery testing. Why is the "right" answer now "no" instead of "yes"? Shesh...