The process to create a CRS881 map is not obvious as the Create function in the M3 interface is greyed out. Rather you need to use the API CRS881MI.AddTranslation call.
From there you can add your mapping information using the M3 client.
While Lawson's MeC tool can access the CRS881 information natively I've recently been using SQL to access this data for interfaces that bypass MeC (e.g. GLS850) - essentially using M3 as a convenient repository for the mapping data and moving it out of the interface code. The SQL tables where the CRS881 information are stored are as follows:
select * from MBMTRN
select * from MBMTRD
I'm now working back through our old interfaces moving the (hard coded!) business logic into CRS881 so end users can manage this data. I'm also beginning to think of ways of (mis)using this program to store data we use for (for example) data warehouse aggregations that are not specified within item / customer master data etc.