When you insert your SD card or USB stick in your Ultimaker, you should see a list of file names when you go into the Print Menu. Printing via USB cable is not supported for any model Ultimaker printer except for the Ultimaker Original.
If you see a message asking you to insert an SD card or USB stick, the printer doesn't recognize what you inserted. This could be due to a problem with the SD card/USB stick or a problem with the reader on the Ulticontroller. Try using another SD card or USB stick. Make sure they are formatted in FAT32. If you continue to experience issues, you can contact Support.
If you see a list of files but the one you want isn't listed, there are a few common causes.
1. The file you want is not actually saved on the SD card/USB stick. Insert the SD card into your computer. Look through the files to ensure the one you want is saved to the card. Cura can save to your computer or onto a removable storage device.
2. The printer does not like the file name. In order for a file to be recognized by the printer, it must start with a letter or a number. If it starts with a special character or a space, the printer will ignore it.
3. The file you saved is not a file type recognized by the printer. The printer runs off gcode files; Ultimaker 3 series printers can also accept gcode.gz files. Gcode files are produced by running your model through a slicing program. If you take a model file from a CAD program or download one from youmagine or thingiverse, your printer won't know what to do with it, because your printer cannot read .STL or .OBJ files. For information about how to slice files you've downloaded and save the to the card, see here.