Admin CP > vBulletin Options > Forum Display Options (forumdisplay) and set Length of Thread Preview Display to a positive number (if it's set to 0 then it is disabled).
Does this function not work if you do because mine doesn't work still do I need to add something to the code?
Firstly, is it enabled in your vBulletin Options? (Refer to Lynne's Post)
If it is then check your threadbit template. AdminCP > Styles & Templates > Style Manager > (From Right pane) Edit Templates > ThreadBit Templates > threadbit
Look for something like the following, it will be near the beginning somewhere, yours may differ slightly due to customisation: