Can I takeback a move if I make a mistake or if there's a mouse slip? (ICCforMac/Windows)

Yes, but only if your opponent agrees. You can request your opponent allow a takeback. They can either grant your takeback or ignore your request.

You are not obligated to grant takebacks even for obvious mouse-slips. You can refuse takebacks in all situations or allow them in some. It is entirely up to you.
To request a takeback from your opponent:
ICC for Mac/Windows: Above your board is a button named 'Takeback'. Click this to request 1 move takeback. On the right of this button is a v which you can click for options 1 move or 2 moves takeback.

The request can be ignored by simply continuing play.

Alternatively you can type 'takeback'  on the command bar under your Console or '/takeback' under your Game chat window.
To accept the offer, your opponent must also type 'takeback'
To ask to take back two moves instead of one, type 'takeback 2' 

 Note: In tournaments run in the Tomato system you can not do takebacks.

For more information read "help takeback"