I need to find a lost 401 k

I want to know how to apply for a withdrawl from my pension or retirement

How to apply for pension and retirement from post office

Call the employer's HR department.

Where did you lose it?

You should be receioving statements on your account regularly - every quarter or at least yearly. If not - perhaps because you moved and faied to provide your new address - call your former employer and ask how to get information on your 401(k) account.