Typically, it is best just to post your code so we can see exactly what you are doing, instead of posting a description of your code for what you think it is doing.
That being said, the matGetNextVariableInfo( ) function just gets header information for the variable without getting the actual data of the variable. It is intended for you to quickly see what is there without loading potentially a very large variable. So seeing NULL behind a subsequent mxGetPr( ) or mxGetData( ) call would be expected. If you want to get the variable along with its data, you need to use matGetNextVariable( ) instead.
