I am trying to upload the CDIF however the BROWSE button is greyed out on the 'Load Data' screen.

Created by Atif Ikramuddin, Modified on Mon, 20 Apr at 8:52 PM by Rohit Sawant

Browse button on the Load Data screen will only be enabled if the Data Load Type is selected which appears exactly above the Browse button.
User first needs to select the Data Load Type i.e., either Delete & Load or Append

After selecting the Data Load type, you need click on browse button and select the required CDIF file and post that click on Upload button.
To get help on choosing the right Data Load Type, please see article - What are the difference between Delete & Load vs. Append data load type? (https://resilinc.freshdesk.com/a/solutions/articles/159000408691?portalId=159000092793).