SQL Workbench/J is a tools for working SQL databases written in Java. You can downloaded it from the official website.
While SQL Workbench has some support for MonetDB, you will still need to download JDBC driver and quickly configure it. After starting SQL Workbench for the first time, it will load its default welcome screen and starts the wizard to select database driver. Cancel this wizard and open the Driver manager, using the menu File->Manage Drivers.
1) There add a new Driver clicking on the button in the top left corner of the dialog and type in a name for the driver, e.g. MonetDB.
2) Click on the folder-like icon and navigate to the previously downloaded JDBC .jar file.
3) The class name will be filled on it's own, but just in case it should be: nl.cwi.monetdb.jdbc.MonetDriver.
4) Type in a sample URL for database connection, e.g. jdbc:monetdb://hostname:50000/database.
Next, to connect to a MonetDB database instance open File->Connect window
1) Type in a name of the connection.
2) Selected the MonetDB driver from the drop-down menu.
3) Enter the details of your database connection, replacing the hostname, port and database name in the sample URL, e.g. jdbc:monetdb://localhost:50000/db for an instance running on localhost, port 50000 with database db.
4) Enter the username and password for the database
Confirm the connection and you are done.