Mating Shaft in Assembly

Hello guys,

i have created the following assembly (is a CSWA sample; i'm training for the exam) but i 'm facing a problem as regards the mating of the shaft.

All other parts are being mated but i can't find how to mate the shaft The shaft is centered in the assembly horizontally (from left to
right) and vertically (between the lower base and upper base).

Any comment is welcomed

Thanks

SolidworksAssemblies