I think you meant 'prøv dette slik'? If so, it's simply more common in spoken danish to use 'den/det her' instead of 'denne/dette'. Both is equally correct though.
Damn, I wish we could save comments on this site for future reference.
If you mean why 'det her' equals 'this', then it's simply a common way in spoken danish to say 'this' instead of 'dette'. If that wasn't the question then I'm sorry.
Why not "Try that candy."? I though "det = that" whereas "dette = this"?