A couple of things that are needed to fully answer your qestion:
- Version of MS Access
- Table structure
But with that said, the item you are looking for is accessible in the Query By Example (QBE) grid.
Unless you are doing somethign very tricky with the data types, then you can just type the name of your 'variable' in the Where section of the grid in the column of the field that you wish to be applying the criteria to:
[*parameter*]
When you run the query, this will cause a pop up window to display that you can put your parameter into and hey presto - all solved.
As mentioned above, you should also read the KB article referenced. You can also use standard form syntax to link the parameter to a form field.
*formname.fieldname*