Am trying to start mining litecoin with litecoin-qt
At some point they removed the GUI for mining from the litecoin-qt client. So you have to run minerd from the command-line or terminal.  The actual command syntax seems to be loosely documented, however. 

Two reasons for this:


cpu mining simply is relatively too slow and unprofitable to warrant much further development. 
Nevertheless the general consensus (at least according to the wiki and other available info) is that pooler/cpuminer is by far a better client in terms of speed, compatibility, usability, etc. 


Also you can pick up a USB ASIC miner for ~$15 (usd) that will far surpass what a CPU can do today. 

As far as the command syntax goes for CPUminer check out
https://bitcointalk.org/index.php?topic=55038.0

And scroll down to basic usage examples. Also may want to register at a pool first and have your login details ready.