You can't be employed and cash it out while you are working.
If you are no longer with them, contact walmart and ask them for your plan administrator.
Or look at your 401K statement and see if it provides a contact number.
Note: Cashing out can easily put you in a higher tax bracket.
Save some money aside for tax time to pay all the taxes and the 10% penalty
(the company may keep 20% aside to help you pay for those taxes)