The ticker symbol for the FTSE MIB 40 stock index is FTMIB. After entering the ticker select Index.
IBKR does not make available real-time index levels for indices published by the FTSE Group. This is due to the disproportionately high licensing fees FTSE would require of IBKR. Affected indices are the UK FTSE 100 and the Italian FTSE MIB 40.
For more information, see FTSE Indices at IBKR in the IBKR Knowledge Base.