Assuming you know the path and it is to a document not stored in Lotus, write whatever text will serve as the link, highlight it (drag mouse with button held down), then go to your menu up top and choose CREATE, HOTSPOT, LINK HOTSPOT. A "Hotspot Resource Link" pop-up window should display. On it, the TYPE should be "URL", and you should enter the path to the document on the VALUE line.
If the attachment is embedded in Lotus already, you should be able to open the document it is embedded in, use the right mouse button to click and pull up a menu, then select "Copy As Link". Return to your memo and paste into the spot the link is to go. A small icon will appear there.