In order for you to log in to your /5 trading account, you will need your account number and password. These details were sent to you when you first opened your account.
Once you have your account number and password, simply install the /5 platform on your PC and select 'File' then 'Log in to Trade Account'. A login box will then appear in which you will be able to enter your account number, password and select the relevant server.
When typing your password, you should ensure that it is entered correctly. If you are copying and pasting, make sure not to copy any extra spaces over.