You can find the tool in a version-specific subfolder of your SQL Server shared features installation directory. For example, look in C:\Program Files\Microsoft SQL Server\110\COM if you've got SQL 2012 installed on the C: drive.
I've always assumed that it's included with Management Studio but recently I found out that's not the case. After testing various installation options I discovered that it's included when the SQL Server Replication features are installed, meaning you also need to install the Database Engine Services to get it:
Although not limited to replication scenarios, Microsoft's documentation hints that it was designed with replication in mind:
The tablediff utility is used to compare the data in two tables for non-convergence, and is particularly useful for troubleshooting non-convergence in a replication topology.Perhaps this is the reason most people don't know about it - replication isn't widely used and if you exclude the feature from your database install you won't get tablediff.
Now you know...