We are now arrived at the happy aera of experimental philosophy; when men, having got into the right path, prosecuted useful knowledge; when their views of nature did honour to them, and the arts received daily improvements; when not private men only, but societies of men, with united zeal, ingenuity and industry, prosecuted their enquiries into the secrets of nature, devoted to no sect or system.
[Colin Maclaurin, An Account of Sir Isaac Newton's Philosophical Discoveries, Bk I, Ch. 3]
Maclaurin is an important element of Hume's philosophical context; one of the best ways to get a general idea of what Hume sees himself as doing in the Treatise of Human Nature is to read Maclaurin's account of Newton's method of philosophy (although the similarities are due to a broader Newtonian environment rather than due to influence), and, perhaps more straightforwardly, it is also clear that Maclaurin is at least a partial source for Cleanthes in the Dialogues Concerning Natural Religion (this can be traced through verbal similarities between what Maclaurin says and what Cleanthes says). Maclaurin was one of the great Newtonians of the time, being himself an excellent mathematician, a friend of Newton himself (who at several points lent a hand in furthering Maclaurin's career), and a member of the Royal Society.
Curiously, though, there's been relatively little close attention paid to the question of the extent of Maclaurin's influence on Hume. As I've noted, we have excellent reason to think there's a connection between Maclaurin's Newtonianism and the Newtonian natural religion of Cleanthes, and there are a few other points. But mostly it's an area of study that's still in fragments.
Charles Twardy has an interesting paper on possible connections between Hume's (limited) knowledge of physics and Maclaurin's account (PDF).
The money passages for the influence of Maclaurin on Hume's Cleanthes are in the Account, Book IV, Chapter 9, Sections 6 & 11 (section 6 starts on page 400 of the edition linked above).
