Learn Turkish: Bütün mesele, projeye değer katmaktır. - The whole issue is to add value to the project.

The -tır is the copula suffix (acting as "is"). In Turkish, an infinitive with -mAk can act as the main predicate of a sentence. You can actually drop the suffix and just end the sentence with ...değer katmak, but adding -tır makes the statement sound more formal, definitive, or like an objective fact, which fits perfectly with a strong phrase like "The whole issue is...".

Why do we use katmak for "to add" instead of eklemek?

While both mean "to add", they are used differently. Eklemek is mostly used for appending something structural or discrete, like adding an item to a list or a paragraph to a text. Katmak is used for blending things together or adding abstract qualities, such as adding flavor (tat katmak) or, as in this sentence, adding value (değer katmak).

Why does proje take the dative case (projeye) here?

The verb phrase değer katmak (to add value) requires the dative case for the entity receiving that value. Because you are adding value to the project, proje needs the dative direction suffix -(y)e.

Why is there a comma right after mesele?