The reason you cannot find the three dots icon is due to a recent interface layout update in Keynote for iPadOS. Apple replaced the older "More" three-dots menu with a dedicated Actions Menu button (which looks like a folder icon, document icon, or a small arrow/dropdown next to the presentation title in the toolbar).
Try this If you want music on your whole presentation
Open your presentation in the Keynote app
Tap the Actions Menu button in the top toolbar (this replaces the old three-dots menu).
Select Presentation Options from the dropdown menu.
Tap Soundtrack.
Tap Choose Audio (or Select Audio) to select a song from your device.
Configure playback: If you want the music to repeat until your slideshow finishes, make sure to tap Loop(you might need to swipe up on the menu pane to see this toggle).
Try this if you want to have music on one specific slide
Navigate to the slide where you want the music.
Tap the Add (+) button at the top of the screen.
Choose Insert from (or tap the media tab) and select your audio file from your iCloud Files or device storage.
Adjust settings: Tap the inserted audio icon, select the Format button (paintbrush icon), and navigate to the Audio tab to set if it plays automatically or requires a tap.