In order to export transactions to a CSV file, you can click on the following icon in the Transactions screen:
It will export whatever transactions are returned from your Filters selection and the CSV file will contain the same columns that are shown in the form that you are viewing fields from (in the example above, the Transaction Form).
However, in order for this functionality to be available to you, you will need to have the proper permission set. Admins, TCs, and ISAs have this permission by default, but it can be granted to an Agent if necessary by going to Admin > Users > Edit (the particular user). Then you will want to select the "Download Transactions" permission and click "Update".
If the user doesn't have this permission checked, they will not be able to export Sisu transactions.