There are two reasons why you might not see it.
If you're just not sure where we mean - click on My Trackers - Start. On this page, on the left column, you should have a block headed "My goals and progress" with how much weight you have to lose, how many calories to eat today, how many to burn today. Beneath those settings are two buttons - the one on the right is "SparkSummary Calendar".
If you don't have "my goals and progress" you may be signed in to the Spark 5 program to maintain a healthy weight rather than to lose weight. Thus you wouldn't have the weight loss settings and goals. If this is the case you can only swap by going to Account/Email Preferences, and hitting Reset My Goals. Note the warning on that.
Otherwise there's a possibility that you have a workplace created account, and they can restrict what tools their members get.
Deb, in New Zealand