I'd recommend using your favorite web browser and going to https://creative.adobe.com/files for uploading from the computer.
Keep in mind tablet capabilities are much less than a computer, so large files might take a long time to download and be very system intensive to work on. Although you can import files up to 12 megapixels I'd recommend something more mid range, like 6 or 7 megapixels, ie 2880 x 2160 or 3072 x 2304.
Also, PSD files are flattened when imported into Photoshop Touch.
Hope that helps,