Help help help

I am - for the x-number time - in the process of installing a sims 3 expantion (never get passed the first, but have all dwl) and now i run into a problem I have absolutly no clue how to fix or convert (from the pc instructions):

2. Now open the Start menu. Type 'regedit' in the search box and press [ENTER]. Navigate to:
HKEY_LOCAL_MACHINE\SOFTWARE\Sims\The Sims 3 and double-click on 'SKU'. Change the shown
number to 2 and press 'OK'. Close the Register Editor

How on earth do I do that?? I tried to just jump it - who knows maybe Mac installation does not need that step, but i get the message about a compatibility problem. All - base and expantions - are game4theworld..

Please help - going completly insane

what operating system are you using which version?

To install the expansion, all you need to do is click on the "The Sims 3 [Expansion Name] Installer" App that can be found within the .rar folder you unzipped. It should boot it up and take you through the process.

I'm not quite sure where you're getting this message that you're reading...
Can you provide more details about what guide you're following?

Are you using Stuffit Expander?