I am not sure which file you downloaded, but I downloaded the Ultiboard Zip file (the small 3KB one). Each footprint is an individual footprint and apparently must be added manually to the database in Ultiboard.
How to do this? In Ultiboard, Extact them to a location on your drive and double click the footprint and it will load onto the screen like a normal board would. Now you have to goto DATABASE>DATABASE MANAGER. Once there you right click on USER DATABASE and add a family like say TRANSITORS. Then highlight your new family by clicking on it. Then goto the very top of the and you will see a little yellow "+" . click that and it will give you the option to add a new part from the current design to the user database. That is all you need. The footprint will be in the user database.
The next step is to assign it to the component in Multisim and you will have to refer to a turtorial on the NI Site in order to do this. I will post a link to it once I find it.
I do not know about the other file. Most of the time there are .PRZ files that the only thing you have to do is to go into the database manager of Multisim and select the user database. This will give you the option to do an import. If your files are a .PRZ file then you will be able to import using this option.
I hope some of this helps a little
