Add Frequency Cap to an interstitial ad (Show ad every X minutes)

Hi there everyone!
I'm new here so I hope I'll have some good treatment from you ^_^ .
AS of right now, I'm dealing with a big problem when it comes to javascript level.

I'm using Adform platform server and I wanted to give frequency capping to an interstitial ad. (I don't have to make the interstital!!!! All I get from Adform is a script like this:

Code:

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<script type="text/javascript" src="http://adx.adform.net/adx/?mid=303469"></script>

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and that's all!

So far I've seen the script made by dynamicdrive, where it checks the cookies and shows or hides content inside divs but it didnt work with that script!

Some help please?!