My suspicion was correct. When you call load with an output argument, that output argument is a struct array with each variable in the MAT-file you load stored as a field of that struct. You just need to pull the data out of the struct.
mydata = load('census.mat')
If I wanted to work with the date and population data from census.mat, I would need to use mydata.cdate and mydata.pop rather than mydata (the whole struct array), cdate (which doesn't exist; the data is in the field of mydata), or pop (which also doesn't exist.)
[p, s, mu] = polyfit(mydata.cdate, mydata.pop, 2);