The Admin tab is only visible to highways user, it is not visible to other users. Please see below in order to enable it.
1. Check by adding these roles to the user : IM_ADMIN, IM_HOME, IM_READONLY, IM_USER. When you have roles granted, log into Information Manager with usual username or password.
If you are unable to assign those roles, then run the scripts from the below wiki, to assign same possibilities to other users:
http://communities.bentley.com/products/assetwise/exor/w/wiki/17009.how-to-make-all-im-acounts-to-look-like-one-of-a-desired-user
or
open HIG1832 and delete all IM% roles for that user and save. Again assign IM% roles to that user and then save. Re-login to IM and check
2. If the above did not work then, manually insert a record into im_user_tabs table for Admin page. Now, the user should be able to see Admin tab.