An alternate row is every other row and you inc 2 sts on them, one at the beg and one at the end of the rows (each end).
You won't reall be increasing in the cable sts, only at the edges of the row, so work the first st, inc, then the rest of the row to the end, inc and work the last stitch. There may be purl sts between the cable sts, so you would work the new ones in purl, then when there's enough, do some in knit. You're building out from the cables so your sts would be something like this....
E inc pp cable.... cable pp inc E (E for edge stitch), then after that it would look like -
E k pp cable.... cable pp k E then later
E kk pp cable .... cable pp kk E
I'd have to know what the pattern is or the cable pattern to be more specific.
sue- knitting heretic