Then click Pages on the left hand side:
Then click on the right hand side Add New Pages and then Request Access to a Page.
On the resulting pop-up, simply type into the box the page name you would like access to, or alternatively, type in the page url.
You will then be given the option of choosing the level of access you require. If you are just requesting the ability to advertise on behalf of the page, then select Advertiser Access.
Then click Request Access.
Once you have done that, any page admins of that page will then receive a notification that your business has requested access. Your request will remain in a pending state, until they grant you privileges.
Once permission has been granted for your business, you will then need to allocate people from your business, to have permission to run the ad campaigns on that page's behalf. From the Pages tab, click the relevant page, and then Click Add People.
The in the resulting pop-up, type in the name of the person you would like to add to the page. Select their level of access, and click Save Changes.
If you are using Driftrock Create for your ad campaigns, then the person that connected your ad account to Create, needs to be the person assigned to the page in Business Manager. So ensure that you have selected them in the box above.