Export/Import to/from JSON and CSV features are under development.
Until these features are available, here is workaround solution how to export to CSV using Robomongo shell:

Workaround Solution: How to export to CSV

1) Create .robomongorc.js file with the following function into your home directory.

(Note: Specials thanks to @jeff-phil for this function: https://github.com/Studio3T/robomongo/issues/348#issuecomment-29362405 and @Ian Newson for the way to do it for aggregates https://stackoverflow.com/questions/16468602/mongoexport-aggregate-export-to-a-csv-file/48928948#48928948 )

// Export to CSV function
function toCSV (deliminator, textQualifier) 
{
    var count = -1;
    var headers = [];
    var data = {};

    var cursor = this;

    deliminator = deliminator == null ? ',' : deliminator;
    textQualifier = textQualifier == null ? '\"' : textQualifier;

    while (cursor.hasNext()) {

        var array = new Array(cursor.next());

        count++;

        for (var index in array[0]) {
            if (headers.indexOf(index) == -1) {
                headers.push(index);
            }
        }

        for (var i = 0; i < array.length; i++) {
            for (var index in array[i]) {
                data[count + '_' + index] = array[i][index];
            }
        }
    }

    var line = '';

    for (var index in headers) {
        line += textQualifier + headers[index] + textQualifier + deliminator;
    }

    line = line.slice(0, -1);
    print(line);

    for (var i = 0; i < count + 1; i++) {

        var line = '';
        var cell = '';
        for (var j = 0; j < headers.length; j++) {
            cell = data[i + '_' + headers[j]];
            if (cell == undefined) cell = '';
            line += textQualifier + cell + textQualifier + deliminator;
        }

        line = line.slice(0, -1);
        print(line);
    }
}
DBQuery.prototype.toCSV=toCSV; //regular find
DBCommandCursor.prototype.toCSV=toCSV; //aggregates

2) Close/open Robomongo to load the script.

3) Example usage:

db.getCollection('csv_coll').find({}).toCSV()

Output on Robomongo shell:

_id,name,surname,age
"584ac96e93775e9ffab70f52","alex","desouza","4"
"58d21b2a8a7c03ab0860fd7d","john","wick","40"
"58d21b378a7c03ab0860fd7e","mike","costanza","44"