SeDuMi is freely distributed and available with source and binaries from http://sedumi.ie.lehigh.edu.
SeDuMi is invoked with sdpsettings('solver','sedumi').
Solver & algorithm documentation
SeDuMi is the most commonly used solver among YALMIP users.
Please note that there are issues with recent versions of MATLAB and 64-bit SeDuMi. Before you use SeDuMi with YALMIP, make sure that your SeDuMi installation actually works, by running some of the examples included in the SeDuMi distribution.