Sorry to hear about the trouble with Lightroom presets. We're here to help.
Which version of Lightroom Mobile are you using? What type of presets have you downloaded? How are you trying to import them, and what happens when you import them to Lightroom? Do you see any error messages?
Does the preset you have downloaded in a ZIP file? If you've purchased a preset pack, it will be in a ZIP file. On some mobile devices, you cannot unzip files without an app.