Maybe what they are telling you to do is to [COLOR=Red][U]make[/U] a yarn-over too,
[/COLOR][COLOR=Blue]in addition to slipping 1 purlwise.
[COLOR=Black]When I stumble upon mysterious pattern directions such as this one you've got...I just make a swatch...and keep tinkering with it until it looks right.
With this pattern you've got...if the row you are having issues with is the row immediately following the cast-on row...then you would not have any pre-existing yarnovers to slip TOGETHER WITH a slipped purl.
So it MUST BE that you are to CREATE a yarnover that you will slip AND you will also slip a Purl.
Does the NEXT ROW ask you to K2Tog over some stitches?
I ask this because those 'yarnovers' in row 1 will be increasing your stitch count.