Though there is choice between web-based (Online) and standalone version (Offline), standalone version is recommended for MultiCharts connection. To download Trader Workstation and Gateway, follow this link:
httphttps://individualswww.interactivebrokers.com/en/control/systemstandaloneindex.php?os=win&ib_entityf=llc14099#tws-software
For FOREX trade size should be equal to the amount of units, not trade lots.