Here is an example for mySQL, but you should be able to follow the steps for mongo db as well: http://www.pegaxchange.com/blog/2016/04/23/accessing-external-mysql-database-from-pega-7-prpc/

You might also want to take a look at this - https://collaborate.pega.com/question/how-pega-chat-can-connect-mongodb-replica-set

Alternatively, you can also use the REST interface to Mongo DB using our Connect-REST interface - https://www.mongodb.com/developer/

This is how I tried:

Placed the mongodb java driver jar in my application lib folder.

Created an activity with below java code:

com.mongodb.MongoClientURI uri = new com.mongodb.MongoClientURI("mongodb://username:password@<Server>:27017/?authSource=Test");
com.mongodb.MongoClient mongo = new com.mongodb.MongoClient (uri);

com.mongodb.DB db = mongo.getDB("Test");

com.mongodb.DBCollection coll = db.getCollection("Test");

oLog.error("Collection Test selected successfully");

com.mongodb.BasicDBObject doc = new com.mongodb.BasicDBObject("Code", 5678).append("name","PegaTesting");

coll.insert(doc);

oLog.error("Document inserted successfully");