Madame Tussauds is in the Grand Canal Shoppes at The Venetian. Any Las Vegas taxi can drop off at The Venetian taxi stand, which is just a short distance away.
From the taxi stand, pass by hotel reception, under the Ponte Rialto (large bridge near the Strip), and cross the street to the left at The Venetian welcome sign. We are just up the moving walkways by the Campanile Tower (brick bell tower) and fountain.