Use a Hyperlink field.
If you want no more than one attachment per record, just add a Hyperlink
field to your table. Insert the hyperlink through the Insert menu, by
right-clicking. Alternatively, you could put a little command button beside
the text box on the form, and put this in the button's Click event
procedure:
Me.[NameOfYourHyperlinkFieldHere].SetFocus
RunCommand acCmdInsertHyperlink
If you may need to have multiple attachments per record, create a related
table with the hyperlinks. Use a subform where the user can insert different
hyperlinks on different rows.
If the idea of hyperlinks is new, see:
Introduction to Hyperlink fields
at:
http://users.bigpond.net.au/abrowne1/casu-09.html