In addition to dk's great idea, you can use comments. while in the desired cell, From the Insert menu or by right clicking, select insert comment. You can put a message here. One thing to remember though is that you will need to set your comments for display. Go to Tools | Options | View and in the comments section, select your desired level. I generally just set it to show indicator. This will put a tiny triangle in top right corner of the cell where the comment is. When the user hovers over the cell, the comment will appear.
You could use this in conjuction with dk's data validation suggestion by setting your comment to something like "click this cell for more instructions".
HTH, Tom