In the TWS OptionTrader or TWS/Mosaic Option Chains window, click on the drop down menu that says SPX in the upper right corner of the option chains. Select SPXW as the Trading Class and the screen will change to the SPXW Weekly options. The rest of the available weekly expiries can be found by clicking on the More tab to the right of the currently listed expiries.
More information on the Option Chains window can be found in our Users' Guide.