They have you place the markers on the purl row (ws) and the next row (increase row) is knit, with a purl row after that.
If you just purled a row, then that was probably the row where you should have placed the markers. You can place them as you knit the increase row, you'll just have to do more counting.
This will leave you on the WS, ready to place the markers.
Place Markers (WS): Work 26 (30)sts, place marker; work 26 (27) sts, place marker, work 46 (54) sts, place marker, work 26 (27) sts, place marker, work last 26 (30)sts.—(150)(168) sts total
Increase Row: K to within 2 sts of marker, increase in next st, K 1, pass marker, increase in next st; repeat between *’s 3 times (8 sts added)
Next Row: K 4, P across to within 4 sts of end, K 4
Repeat these two rows 4 (5) more times. (190 )(216) sts, putting sleeves on holder on the last (WS) row. Count ridges. 3rd buttonhole is coming up!