I have the trial version of ArcGIS 10 (ArcEditor licence). The trial automatically installs and authorises all extensions for the trial period. However, in ArcMap I can only see Maplex, 3D Analyst and Spatial Analyst in my Extensions dialog box. When I check ArcGIS Administrator, it lists all the extensions as "authorised", but only the extensions I mentioned above as "installed". The others are "not installed". I am a bit confused about this, as the extensions are supposed to be automatically included as indicated here.
Ah - just found the answer here,by going to Control Panel and modifying the current ArcGIS 10 installation to include the extensions, although why they are not installed by default I do not understand.