To upload the creative within Pexi you need a Pexi account and access to the correct advertiser. You can request a Pexi account via firstname.lastname@example.org. When you have access to Pexi you can upload the banner as a zip (zip the folder, not the files). If you include the IAB size within the filename Pexi will automatically detect and adjust the upload settings.
A popup is a rectangle sized creative that sits at the bottom right of the page above all the other content. Users can choose to hide the popup with the use of a close button.