Hi there,
In design view of your table, goto the menubar and Select "View", then select "Indexes". This dialog box lists all indexes in the table.
To set two fields as being unique, do the following:
1. In the "Index Name" column, type a description of the index e.g. "TwoColumns".
2. In the "FieldName" column, select the first field of the index
3. In the next row below leave the "Index Name" field blank, but select the second field of the index.
4. Click back on your index name, e.g. "TwoColumns" and at the buttom of the dialog box there should be a field called "Unique". Change this value to "Yes"
5. Save your table
All done,
Hope this helps.
Cheers,
Dan