What is the stress pattern of Macbeth's soliloquy in act 2, scene 1?

The speech, as favoritethings's answer states, is in iambic pentameter, with only a few alterations to the meter. These alterations begin entirely conventional, not the kind of unusual alterations that Shakespeare often uses to suggest a stirred-up state of mind. As the speech continues, however, things get more irregular.
The first irregularity is early on:

a DAGger OF THE MIND, a FALSE, creATion

This line has 11 syllables, with the 11th unemphasized. This is known as a feminine ending. It's very common in Shakespeare and should not be taken as an indication of anything unusual, unless a speech uses a series of them in a row, which this one does not. There is another feminine ending at "senses."
The next irregularity is a more unusual, a short line, only 6 syllables:

as THIS which NOW i DRAW

[da DUM da DUM is missing here]
This short pause may give a moment for Macbeth to contemplatively draw his dagger without speaking.
Another very conventional type of irregularity in meter is called an "initial trochee," where the first two syllables of a line are emphasized, "DUM-da," instead of "da-DUM":

NATure seems DEAD, and WICKed DREAMS aBuse

Because the word nature is emphasized on the first syllable, the line starts with a trochee instead of an iamb. This is a very conventional irregularity and not to be over-interpreted.
Right after this, though, things get more interesting. We have the line:

The curtained sleep. Witchcraft celebrates.

One might initially scan this as follows:

the CURtained SLEEP. witchCRAFT celEbrates

However, there are only 9 syllables here, and "witchcraft" and "celebrates" are both mis-emphasized. What is going here is that there's a missing syllable at the middle of the line. This is called a broken-backed line and is scanned as follows:

the CURtained SLEEP [da] WITCHcraft CELeBRATES

This makes it a regular line. Broken-backed lines are very unusual and suggest something strange is happening with the character. It's up to you to interpret why this happens at this exact moment, though!
The rest of the speech is regular, with the exception of one feminine ending on "murder." If anything, it's surprising how regular Macbeth's meter is, given the complexity of his emotions at this moment. What do you make of that? Why do you think the more unusual irregularities come at the moments in which they do?

There are a few lines that are not regular, strictly speaking, but the soliloquy itself is, for the most part, comprised of speech in blank verse: unrhymed iambic pentameter.  This means that each line has five feet, each foot consisting of two syllables: one unstressed (or unaccented) syllable followed by one stressed (or accented) syllable.  I will try to show the stresses by capitalizing stressed syllables in the words of the quotation below.  The end of each foot is marked off by a slash.

I HAVE / thee NOT / and YET / I SEE / thee STILL. /Art THOU / not, FAtal / VIsion / SENsi / ble (EM syllable)To FEE / ling AS / to SIGHT? / Or ART / thou BUT /A DAG / ger OF / the MIND, / a FALSE / CreA / tion (EM syllable)ProCEE / ding FROM / the HEAT / OpPRESS / ed BRAIN? /

I've used the phrase "EM syllable" to indicate that two lines end with an extrametrical syllable: an additional unstressed (or unaccented) syllable at the very end of the line.  It does seem to make sense, however, that this soliloquy would have so many lines that are irregular (like the lines with the extrametrical syllables) because Macbeth's state of mind is disturbed.  He is actually hallucinating the murder weapon here, and so his speech seems to be a bit confused, just as his thoughts are.