shiftmatrixPWM and shiftPWM are similar but they do different things.shiftPWM controls a daisy chain of ShiftRegisters(SR's) to allow you to use software PWM on their outputs.shiftmatrixPWM controls two chains of SR's. 1 set for vertical and 1 set for horizontal in a Matrix configuration. It handles all the row shifting and allows PWM on all LEDs in the matrix.As far as I am aware ElcoJacobs is still working on the Matrix version and he hasn't put it on the site yet. Only place you can get it is from the forum posted linked by OP.I have used both but in the 0023 IDE. It seems you are using a newer IDE and I am not sure if this has been tested. I am assuming you are trying to compile the Example in the library?I would close the IDE, remove the library, re download the library and extract it again to your library folder. Just to be sure there are no errors and then trying to compile the example sketch.