You can take Metro Line 2 under Hongqiao and get off at Yaohua Road Station in People's Square. Line 8 is the main entrance of Expo 6, and you can see the China Pavilion. Now you can only see the China Pavilion.
Get off the train station, take 1 line People's Square and turn to Line 8. Same as above, you can directly see the Expo Axis and China Pavilion at the entrance and exit of Hua Yao Road.
The extended weekend is quite crowded. It seems that the rules of tickets have changed a little recently, so you'd better call for advice.