See the following links
http://help.sketchup.com/en/article/3000177[quote=“vstapelbsa, post:5, topic:12417”]
it only shows installed extensions such as Ruby etc…but NO button at all!
[/quote]
Evidently you are using the first release of SU8. The Install Extension button was added in Maintenance release 1 or 2. (So long ago, I forget.)
Of course it is 2016 now. SketchUp 8 hasn’t been supported for a number of years and you may be out of luck trying to get a later release of it. Even if you do, SketchUp 8 will not access the 3D Warehouse or other web-based resources as mentioned in the links, above.