I have a little time on my hands...Let me try to show you what Pat is saying to do.
Create a form based off of the table that stores your part number descriptions. Put all the fields on your form you need to show. i.e. part description.
Show your toolbox and find the button that says "combobox". Drag it over and put it on the form. A wizard will start. You want to select Option #3 "Find records that match my selection". Your combo can be based off of any table or query that stores the Unique value to identify your partno. Based on what you have said thus far, I'm guessing you have it in the same table as the form you just built. You should be sure to include the PartNo as one of the fields you want in your combobox.
When you are finished, goto form view and use your combo. When you select the part no from this list, the fields below should reflect the description related to that part no. You can also begin to type in the part no and access will fill it out as you type.
This automation allows you to store much more information. Dlookup is used for display only. Say if you wanted to show a piece of information from another table not used in your form or combo.
If you still want to use it, it goes basically like this.
Unbound textbox is:
=Dlookup("NameOfFieldInTableYouNeed","NameOfTableYouNeedInfoFrom","[FieldNameThatMatchesForm]=Forms![FormNameYouAreOn]![FieldNameThatMatchesTable]")
