Clarification what exactly this question wants regarding force I would like to ask you if the question asked 

What is the minimum force parallel to the plane, which can be prevent the block from slipping down the plane?

I just did the calculation and and got the answer correctly but I didnt understand why i calculate like that to get the minimum force for that situation.  And also, for what situation should I use static friction coefficient and kinetic friction coefficient? That's all thanks. I am studying for my final now so I need clarification not just by doing it right, but also knowing what I'm doing. I'm hoping for further explanation. :)
So basically this is the plane and the block.(The drawing is quite epic as I'm using 'Paint'.)

 A: Kinetic friction applies only when there is relative motion between the two objects.  The phrase "to keep from slipping" implies that the object is at rest: no relative motion.  We are in the realm of static friction.  
Static friction provides an upper limit to the magnitude of the force that friction can provide.  The force due to friction can be very small if the angle is very small, but it cannot be very large.  At some point, as you increase the angle, the force required to maintain static equilibrium (keep the object from slipping) will exceed $\mu_s |\vec{N}|$.
In your case the angle is fixed in advance.  Presumably, in this case, the object will slip if no additional forces are present, such as from your finger pushing up-ramp on the object.  You need to supply an additional force to counter the portion of the force of gravity that exceeds the maximum force allowed by static friction.  Evidently, you have done the calculation correctly.   This new applied force is a minimum, because you can if you wish apply a greater force with your finger, countering even more of the gravitational force.  You can continue increasing the force of your finger until the limit of the static friction is exceeded in the upwards direction:  the force delivered by your finger then exceeds the force due to gravity by $\mu_s |\vec{N}|$.  Hope that helps. 
