Indices per se cannot be traded. However, you can trade products whose value is related to an index, such as ETFs, futures, options, CFDs on the index. Once you type in the symbol, the contract box will open and allow you to choose the product you wish to trade. Click on Stock, Futures, Futures Options, Options or CFD (all when applicable) in order to select the certain contract you wish to trade.
See also: How can I find the MEXEM trading symbol?