My guess is that Image Processing Toolbox is not installed.
imshow used to be part of the Image Processing Toolbox, and was moved into base MATLAB for the R2104b release. You can see mention of this in the "Graphics" section of theR2104b release notes. So, to use imshow in R2014a and earlier, you would need Image Processing Toolbox in addition to MATLAB.
If you are unsure what products you have installed, type: