Go to Orders -> All and on the right most side, near the Create Order button you will find two white buttons. Import and Export.
Before Importing Orders you first need to download an example .csv file, so that you would know how the table should look. You can get the most up-to-date .csv format by simple exporting a single order.
Simply mark a checkbox for one order, the press Export and select Export Selected from the dropdown.
Or by clicking Import Orders and pressing Download Example Import Sheet
Open the now downloaded sheet and fill in the fields as per example (try to fill in as many fields as possible). After you're done, simply press Save and go back to Multiorders Orders page.
Once here, press Import and then press Upload Sheet, find the .csv file you just saved and press Import.
Depending on the volume of orders this could take a few seconds or a few minutes. If however the upload has completed and the new orders did not appear, there may be a problem in the way you filled out the .csv sheet. Contact customer service in this case and they will be able to fix your sheet.