Sennheiser PC headsets have dual 3.5 mm jacks (one for the headphone sound and one for the microphone) and both cables need to be connected for functionality. These headsets can work with a Mac however it is a matter of connecting the headset correctly and then selecting the headset correctly within the sound/audio settings as the default playback and/or communications device.
If the computer has the dual jacks (one for mic and one for headset) then connect both cables and select the headset in the computer audio settings and have functionality.
If the computer only has a single jack you will need to determine if it is a headphone only jack or a dual headphone and microphone jack and depending on the type of jack it is will determine connectivity.
If the computer has a dual jack (ie: accepts both mic in signals and provides audio out signals) then an adapter to convert the dual plugs on the PC headset into a single connection will be needed. Sennheiser offers the the PCV 05 accessory as an option for this. Once the adapter is in place and the headset is connected to the computer then it is just a matter of selecting the headset in the computer audio settings.
If the single jack is a headphone only jack then it will not accept a microphone input and you cannot connect the headset in there. Instead the headset would need to connect via a different input (like a USB) using an adapter. Once the appropriate adapter is in place and the headset is selected in the computer audio settings then the headset will function correctly.