If you can't find any and if you don't quite understand it, then join the crowd, because even the greatest of critics (see below) differ as to whether or not there are puns in the play at all.
In his other plays, Shakespeare is the king of puns, and he uses them not only for comic effect but as equivocal (deliberately ambiguous) language to heighten the mood of a scene.
The book The Players' Shakespeare: Macbeth reveals the following ways in which puns are used in Shakespeare's plays:
1) to show off intellectual brilliance and verbal dexterity
2) as a simple jest to underscore a point
3) to sharpen the irony of an aside or soliloquy
4) a flash of bitter insight
5) as part of an exchange of witticisms
The Porter soliloquy is often said to be "comic relief," but most good critics will tell you it is not. They will say the scene is as dark and foreboding as any other scene, that it actually is a mini-version of the play itself, that Macbeth is the "equivocator," that the scene gives Shakespeare's definitive version of hell. When asked, note critic A. C. Bradley's response:
Q: Is Porter in the play for comic relief?
A: Not really; his speeches are more grotesque than humorous.
So, the only pun that I can see comes after the soliloquy, when Macduff and Lennox enter. The Porter says to Macduff:
"Made a shift to cast" could be a pun for "managed to vomit," but it's about as oblique a pun as I've seen.
Famous critic Samuel Taylor Coleridge in his "Notes on Macbeth," says that Macbeth is so "wholly tragic," that it is absent of any puns:
I merely mention the fact of the absence of any puns in Macbeth, as justifying a candid doubt at least, whether even in these figures of speech and fanciful modifications of language, Shakespeare may not have followed rules an principles that merit and would stand the test of philosophic examination. And hence, also, there is an entire absence of comedy, nay even of irony and philosophic examination in Macbeth,--the play being wholly and purely tragic.
And there are those who don't believe Shakespeare wrote the Porter scene at all, that it was written by some mob of actors, themselves drunk.
Shakespeare's plays are full of puns. His audience enjoyed a dirty joke, much like audiences today. Not all of his puns were sexual in nature, but most of them were, or were at least off-color -- even if they seem tame by modern standards.
The porter's pun has to do with the fact that he is very drunk and must urinate because of the large quantity of ale he has consumed. Look again at the scene and see if you understand it better now.