Using plugin to update Users

I have a closed User Group with the following column headers:
Username, Name, Email, Role, Posts, Last Login, First Name, Last Name, Membership Number.
As far as I can see the Name Column is the addition of the First Name and Last Name columns with a space between. The Last Login column is not used. The Membership Number Column is vital. The password column is not shown.
I have some 545 members to enter using a .csv file in which all details except Posts and Last Login, but including password, are shown.
However how do I match those column headers with the headers shown in the sample file “user_login”,”user_email”,”user_pass”,”first_name”,”last_name”,”display_name”,”role”,”custom_usermeta_1″,”custom_usermeta_2″?
I match Username to user_login, Email to user_email, Password to user_pass, First Name to first_name, Last Name to last_name, but what do I match Membership Number to?
When I try to upload the details appropriately matched with Membership Number to custom_usermeta_1 I get an error message, but when I click on the link I cannot find the error message, so I have no idea what I am doing wrong.
I would greatly appreciate some assistance. I am a relative newbie but am trying to avoid having to make 545 odd individual entries!
Thanks, David