Where do you miss the button? I can see it correctly (iOS app).

Note - an activity needs to be public to be share-able.

Yes I use IOS. My activities are not private for my friends. I want to share my activity on instagram but i cannot find the share button.

Try setting the activity to public (everyone). The sharing button will then appear under the activity (next to the kudos and comments buttons).

Now everyone can see my activities but i still not can share anything 

Can you post a screenshot perhaps?

The button should be in the bottom right.