Normally if your form is linked to the query you can refer to any of the values in your query on your form. However, assuming in this case you have linked the form to another table or query and you want to pull through this one value from this other query, then on your form create an unbound text box and use the following control source :
Be sure to use the same syntax DLookup("[...]","...") and your actual field name from the query as well as the name of the query. If your query is a totals query, then make sure you get the right name (e.g. MaxOf... or SumOf... etc.) If you are trying to get the value from a table instead of a query, then use the table name where I used a query name.
Hope that helps, Andrew