as far as i know, in spms full factorial design you have to define the 2 of
your factors by adding a "new factor" per each. enter name and level of
factor per each new factor.
in the second step you must define cells per each factor and level, e.g. 4
new cells in a 2x2 design. the you should define for each cell the levels
like [1 2] is the second level of the first factor and asign the related
scans fpr this cell.
this should be the major part. you can add additional covariates as
nuisance variables if you like to.
hope this helps (and hope this was correct)