Try the front panel button labeled Source; it's the second button from the left. (The first button is the Power button.) All of the buttons are on the right hand side of the bottom frame of the TV. Use the Channel Up/Down arrows to navigate through the list of inputs. Try waiting for the selection to take effect. (The documentation says press OK or the Volume Up button. Sometimes, the Source button on the TV functions as an OK button. This isn't clearly identified for the Palsonic.)
If the HDMI input is greyed out, the TV may not be seeing a compatible signal from the source. Try turning on the device when the TV is off, then turn on the TV. I'm not sure if this TV will correctly see the signal from a computer. However, on some laptops, you'll need to enable the external video first. This is disabled to save power. Then turn off the computer. Finally, turn on the TV before turning the computer back on.
Palsonic has the manual available for the TFTV492LED on the product (discontinued) site under the Downloads tab. See http://palsonic.com.au/archive/led-tv/tftv492led.htm
. The direct link to the manual is http://palsonic.com.au/customer_service/led-instruct/Palsonic_TFTV492LED_Manual.pdf
I hope this helps.