MyGUI use Multilist        
Multilist filling example code
const MyGUI::IntSize & size = mMultilist->getClientCoord().size();
mMultilist->addColumn("Key", size.width / 2);
mMultilist->addColumn("Value", size.width / 2);

for (VectorStringPairs::iterator iterProperty = widgetContainer->mUserString.begin(); iterProperty != widgetContainer->mUserString.end(); ++iterProperty)
   // add new line and set first element (column 0, aka "Key") in this line
   // set item at 1st column (aka "Value") last line (mMultilist->getItemCount() - 1)
   mMultilist->setSubItemNameAt(1, mMultilist->getItemCount() - 1, iterProperty->second);

So addItem adds a line and 1st element of it. To set something in other columns you should use setSubItemNameAt.