Sigasi 2.x is shipped with only the IEEE libraries that are defined in the VHDL 93 standard (IEEE 1076.3). If you want to use a more recent standard or another version of these library files you can follow this procedure:

1. remove the IEEE folder from your project
2. open your the folder with your IEEE files in your file browser.
3. drag the folder to your Sigasi project to create a link to your IEEE folder.
4. map the new IEEE folder to the IEEE library: right-click > Library Mapping > ieee

If you want to restore your standard libraries, right-click on the project > Library Mapping > Restore Common Libraries

See how this works in a screencast.