You haven't said what is failing or provided error messages... so I'll answer blind.
The general sequence would be something like this:
bk edit BitKeeper/etc/config
bk citool --> enter comments for both file and changeset then click commit
Command line only, you would use the following instead of
bk delta -y"file comment" BitKeeper/etc/config
bk commit -y"changeset comment"
If you do not supply the -y option, you will be prompted for comments.
Of course, this all assumes you want to make this a changeset. For example, if you just wanted to make a personal preference configuration change, I would suggest that you put that into
bk help config-etc for more info.