15th, Feb, 2011: Problem Solved

After few days of struggle I've finally managed to link the entry box of class number in the input data form to a selection of class numbers generated in another form separately. I've learned to create relationships in between tables, and also in order for the program to look for existed values in another table/query when inputting new data, I'd go to design view of the table and hit the lookup tab when the box is selected. Set 'Display Control' to List Box, 'Row Source Type' to Table/Query, and 'Row Source' to the table/query you want the values to be looked up from.