There are a few different ways to enter invoices in AX depending on the processes you are trying to run. As well if you are doing non PO invoices or purchase order based invoices. If you are doing purchase base invoices then you can use the pending invoices process to enter the invoices. Here is a quick walk through of the concept of the pending vendor invoice.
You can use the pending invoice process as well for non-PO based invoices if you use a category at the line level or a non-stocked item. You can see more on this previous post.
As well if you are just entering expense type invoices then you can use for example the invoice register. You can see more on this in the previous post .