You've got a basic one-to-many relationship so I'm not sure what the problem is. One customer can have many quotations, so you need to store the Customer.Key in the Quotations table..
Have you defined this relationship in Tools>Relationships? Is this form only showing the Quotations table, or is it a subform of the Customer form?
If it's a subform, check your Master/Child fields to make sure they're correct. Your subform should NOT have any fields from Customer.Key on it. It should have the Long Integer field that stores Customer.Key in Quotation.CustomerKey(FK).
If this form is Quotations by itself, try making the field a combobox, or with the lookup wizard. It should look up in Customer.Key but store in Quotation.Key.
Hope that made sense. Post back if I've made things worse.