The Scottish Nationalists currently hold 2 seats in the European Parliament. But they've done well in recent elections -- they scored 44% of the vote in the 2011 Scottish Parliamentary election, and 33% of first preferences in the local elections a year later -- so they are probably hoping to win 3 seats in this European election.
Having said that, the SNP probably view the European election as somewhat of a sideshow. Their raison d'être is Scottish independence, and there will be a referendum on that a few months after the European election. They'll be hoping for a good result in the European election that will transform into a win for the Yes vote on independence.
The SNP will also probably be secretly hoping the Conservatives do well in the European election. Many Scots don't like the Tories, and the prospect of another Tory government after the 2015 UK general election could push a few into the Yes camp for independence.