If you live in Scotland, you’re almost certainly registered to vote – practically everyone was in the referendum in September 2014, and if you haven’t changed addresses since then, you still are.
If you didn’t receive a polling card because you registered too close to the 20th April deadline, or for some other reason, you can still vote: find which is your local polling station from your local election office and just go there. You don’t need ID to vote, but if you don’t have a polling card it would be a wise precaution to take some proof of address/identity with you: a local polling officer may not know the regulations and it would be faster to provide your proof than to argue with them. (You shouldn’t have to argue with them and you probably won’t, even if you don’t have a polling card: please don’t go in with a confrontational attitude.)
If not for the rise of the SNP, Labour would be looking forward to five years in government, the Conservatives would be lagging behind Labour by 30+ MPs, and the LibDems would be looking forward to five years as the party that helps either Labour or the Tories form a majority government.
If not for the rise of the SNP, Labour would be comfortably the largest party in the House of Commons after 7th May instead of desperately trying to save what they can in Scotland, and the LibDems might be expecting to lose only half their seats, instead of a likely two-thirds.