Installation of a Simutrans-Experimental pakset is the same as of a Simutrans-Standard pakset. You need to unpack the pakset to your ../simutrans/ directory.
Suppose, for example, you are running Windows, and have Simutrans installed in C:\Program Files\Games\Simutrans, and you want to install Pak128.Britain-Ex.0.8.4. You need to unpack it to C:\Program Files\Games\Simutrans\Pak128.Britain-Ex-0.8.4.
For Linux, it would be something like: /usr/share/games/simutrans-experimental/simutrans/Pak128.Britain-Ex-0.8.4 (depending on your distribution).