Use the Clinical Queries menu on the Advanced Search:
There is no easy way to specify quantitative studies in CINAHL.
Randomized Controlled Trials (check box or in the Publication Type menu) are often quantitative, or at least have a quantitative aspect.
Use the Publication Type menu:
These don't guarantee quantitative studies, but they help narrow things down.
If you want to limit your search to quantitative studies, first try "quantitative" as a keyword (example: lactation and quantitative).
Other ways are to use methods as keywords. Such as :