The previous answer provides you with excellent advice, but I would also advise that you check with your teacher about this type of citation. Several versions of MLA format no longer require Roman numerals; so your citation could appear as follows:
"Know this is a joyful trouble to you" (2.3.47).
Notice that for plays with line numbers, you include the act number first, then the scene (if one is available), and then the line or line numbers. The period goes outside the closing parenthesis. Be sure to include some type of lead-in to quotations so that they flow smoothly with the rest of your paragraph.
I know that this can be a frustrating process because there are several conflicting versions of the same format (MLA in this case). So, it really is best to talk to your teacher/professor about what he/she prefers.
The answer to this sort of question is right at your fingertips. Just google "MLA format" and find a good site (such as OWL at Purdue or the link below, which is full of Shakespeare examples) that details how to cite a quotation from a play in MLA format.
Here's the answer:
Macbeth includes the phrase “Know this is a joyful trouble to you” (II.iii.**add line number or numbers here, using normal aka Arabic numerals**).
Page numbers aren't used in MLA citation of plays that have lines. The "II" in the parenthetical reference above is the Roman numeral for 2 and is made up of two capital "i"s. Remember to include a Works Cited entry for the play as well.