The matlab code that I use in lectures can be found at
http://cs-www.cs.yale.edu/homes/spielman/462/resources/
The graphs that you need for problem set 1 may be found at
http://cs-www.cs.yale.edu/homes/spielman/462/graphs/
Each graph has been save in two formats:
.mat is a file you can load into matlab. The adjacency matrix of the graph is called "a". The variable "note" states the origin of the graph.
.edges is a plain text file with a list of edges. Each line has two integers giving the numbers of the vertices in an edge. The first line of the file begins with #, and is a comment indicating the origin of the graph. These files are probably compressed with gzip.
Note that these graphs are not necessarily connected.