Only Employers can log in to the ESI website http://www.esic.in
and search the IP Number details.
If you are a HR staff or employer , you can get email id from ESI , which is being issued as part of Smart card introduction.
The user id is issued at a meeting conducted by ESI for employers.
Once you log in to this site , you can make use of the PDF Help document that gives all details including the screen shots for searching IP of employees.