Changing number to negative based on another cell value

I am fairly proficient with Excel, but new to VBA and Macros. I tried to cut/paste the code from the following thread and changed the values based on my data, but I cant get it to work.

I have a dependent pull down in column B, (based on a table in sheet 3) that changes what choices you get in column C. The dollar figures are in column F. I'm trying to do the same as the OP, where if I select "Withdrawals" or "Liabilities" it will automatically convert the dollar figure in F to a negative. Please help me from losing more hair!