Is it ok to mention assistance in grant writing on your CV? On behalf of my advisor, I recently wrote a grant to obtain some specialized and expensive hardware. Is it ok to mention this on my CV, even though the grant is in my advisor's name and if yes, what would be a good way to word it ?
You can list on your CV whatever you think is useful information for the reader. In your case, whether something is useful depends on what your position in life is. If you're a full professor with a long history of funded research, what you describe is likely not useful to list on a CV. If you're a graduate student with an otherwise relatively short CV, then that's a different story. I would suggest wording such as 


  Co-authored the proposal for grant XY-1234-5678 (PI: Professor Z).


Now, whether an entry such as this has any impact is a different issue, but it is certainly not going to hurt.