It has been such a long time we have not go to Pasar Seni, I know can get there taking KTM or Putra LRT.
If you want to go there by Putra LRT , you can take LRT at Taman Jaya which is opposite of Amcorp Mall. The price of a one way ticket is $2.
From there you will reach Pasar Seni and it is very near to Petaling Street. You can reach there by walking.