Follow the instructions on this page to limit your Nexis Uni search to a particular publication. You can also look the publication up in BU Library Search and follow the link to Nexis Uni. That will get you to the same place as these instructions.
Searching Individual Sources
To limit your search to a particular publication:
Select All Sources from the Menu drop-down on the Nexis Uni home page
Search for your source, click to select it, then click on the magnifying glass icon.
Click the "Add All These as Search Filters" button. (Note that you can search for multiple sources and add them all at once.)
Enter your search terms in the search box at the top of the screen.