To change the external viewer settings, go to Options -> Preferences -> External programs.
By default the entry table will contain two columns with icons for those entries that link to external files and URLs. You can disable any of these in Preferences -> Table. The second column of the entry table shows icons for PDF or PS files (only PDF if both are present), and the third column shows icons for URL or DOI (only URL if both are present). You open the file or web page by clicking on an icon.
Note: If the PS field does not appear in the entry editor (in the 'General' panel), you may have to customize the entry types of the general fields.
Additionally, if you give your PDF and PS files names that correspond to the relevant entry's bibtex key (plus '.pdf' or '.ps'), JabRef will be able to search your main directories and its subfolders for the correct PDF and PS. Once the correctly-named PDF or PS is in place you access this function by clicking the 'Auto' button next to the PDF and PS fields in the entry editor. If the PDF or PS is found, the field will immediately be set.
If you name a PDF or PS file as mentioned, you can also open the file without setting the PDF or PS field of the entry at all. The downside is that the PDF or PS icon will not appear in the table unless you do (manually or with the 'Auto' button).
You can set PDF and PS directories specific to a database (File -> Database properties). These directories will supersede the main directories.