If you need to enter invoices for expenses in AX you can use a few different approaches depending on how much data you want to collect and if you are using inventory or need a receiving matching process. If you are just in need of entering an expense invoice then you can use the Accounts payable invoice journal. This is a quick walk through.
PS. I mentioned in the video about another video on how the accounts payable account is defaulted. You can see this setup here