The steps for building and running custom MapReduce applications that run against MapR-DB are the same as the steps for building and running custom MapReduce applications that run against Apache HBase. The steps are documented in the Apache HBase Reference Guide at http://hbase.apache.org/book.html#mapreduce.
However, you must use an HBase JAR file from MapR's Maven repository at http://repository.mapr.com/nexus/content/groups/mapr-public/org/apache/hbase/hbase-server/. The name of the JAR file that you use must contain the version of HBase and the version of MapR that you are using.
For example, if you are using HBase 0.98.9 and MapR version 4.1, you would use the file