You can either nominate beneficiaries online in the "About You" section, within "Your Profile", or complete a Nomination of beneficiaries form. 

To nominate a trust or charity please complete a Nomination of beneficiaries form, as you can't do this online.

By nominating beneficiaries, you are requesting that the administrator of your pension scheme takes your nominations in to account when deciding how any remaining pension savings are distributed on your death. This isn't binding on the scheme administrator, but they will take your nominations into consideration when making their decision. You don't have to nominate beneficiaries, but it does help the scheme administrator if you do.