Is the text-link field broken in D7 views integration?
I think the fix is in 4.7 - I have the same problem in 4.6.10.

There is a workaround - you can set any of the other fields as a link to the activity.

In your chosen field (I used the Activity Subject), go to 'Rewrite results' and select 'Output this field as a link'. In the Link path field, add:

civicrm/activity?action=view&amp;reset=1&amp;id=[id]&amp;cid=!1

Then apply the change and save your view. This gives a link to the activity - if you want an edit link, try:

civicrm/activity/add?reset=1&amp;atype=&amp;action=update&amp;reset=1&amp;id=[id]&amp;cid=!1&amp;context=activity

Views will replace [id] with the activity ID and !1 with the contact ID

(If you want a text link separate from the other fields, add a Global custom text field and set it to rewrite as a link)