Create a new data field using set

$set example

> var d = {
... drives:{
...   C:{
...       windows:{
...           'explorer':{
...                 admin:'r+w'
...           }
...       }
...   }
... }
... }
> db.drives.save(d)
> db.drives.findOne()
{
        "_id" : ObjectId("4d6fd353af4a000000006c1f"),
        "drives" : {
                "C" : {
                        "windows" : {
                                "explorer" : {
                                        "admin" : "r+w"
                                }
                        }
                }
        }
}
> db.drives.update( { _id : d._id }, { $set : { "drives.C.windows.explorer.admin" : "r" } } )
> db.drives.findOne()
{
        "_id" : ObjectId("4d6fd353af4a000000006c1f"),
        "drives" : {
                "C" : {
                        "windows" : {
                                "explorer" : {
                                        "admin" : "r"
                                }
                        }
                }
        }
}
> db.drives.update( { _id : d._id }, { $set : { "drives.C.windows.explorer.guest" : "e" } } )
> db.drives.findOne()
{
        "_id" : ObjectId("4d6fd353af4a000000006c1f"),
        "drives" : {
                "C" : {
                        "windows" : {
                                "explorer" : {
                                        "admin" : "r",
                                        "guest" : "e"
                                }
                        }
                }
        }
}