JMdict: Getting Started
First, read the Getting Started page in the database documentation, so that you are aware of the basic field types, tags, etc.
Read the editorial policy page, which has a lot of general information and rules on constructing and formatting entries.
Once you are ready to put in an entry, go to the New Entry Screen. Select "JMdict" in the pull-down menu labelled Corpus.
Entries can then be keyed into the appropriate boxes.
PLEASE put some reference details. Your proposed entry will be checked by one or more editors, and if you don't give them some supporting information, they will get grumpy.
Sign your entry, even if it's just a pen-name. Why? Well it will help the editors if they can identify your entries as coming from one person. Once you establish a track-record as a reliable source, they will be more trusting and less picky about what you submit.
Note you don't need a user account or to be logged in to enter entries. Only editors log in at present.