To install, go to the 'Portal' foler which has all the portal data in it (maps, bin, cfg, gamestate, e.t.c.).

Now open, in a separate folder, the 'Colkadome_Isolation' folder you just downloaded.

Drag the contents of the folders (or the folders, if it wont replace anything) in the 'Colkadome_Isolation' folder to the folders with the same name in the 'Portal' folder.

E.g. You should drag the 'Colkadome' folder in the 'maps' folder, into the 'maps' folder within the 'Portal' folder.

Make sure all the important-looking files have been correctly placed in the 'Portal' folder, run Portal, go to 'Bonus maps', and find the new maps!