How can I add a quota to users in Collaborative Forecasts.
In the AppExchange, Salesforce labs has made available a free package to help with this: Edit Quotas. It creates an extra tab that allows you to enter quotas for users, for periods, manually. Much easier to use than Data Loader.
You can upload quotas to the RevenueForecast object via the Data Loader. Features coming in Spring14 may change that.