You can't. It's an inexact science at best, and the engines keep their data very private.
Read (or reread) the thread I link to in my post above.... It's all there. Here's a partial list, spelled out in more detail.
a) demographics - The engines have very different audiences. WordTracker pulls its data from some pretty lowbrow metasearch engines, and then assumes it can get the figures for Google just by adjusting for market share. It doesn't work.
b) sample size - The metasearch engines that WordTracker uses have a very small sample size. WordTracker extrapolates this small amount of data by multiplying it up to represent a much larger share of web searches. In so doing, they're also scaling up their errors.
Say their metasearch data is one-fourth of one-percent of all web searches. Say Google is fifty-percent of all web searches. In multiplying their figures by 200, which is what they'd have to do to predict Google traffic, they're also multiplying their errors by 200. A difference of 1 or 2 searches in WordTracker's data shows up as a difference of 200 or 400 searches in their predicted Google searches.
c) machine automated searches - These skew all the engines, but most significantly skew Overture. Let's say somebody checks his position 4 times a day. That's 120 times a month. If ten people do that, that's 1200.
WordTracker also occasionally shows automated searches... and these get scaled up unrealistically when they extrapolate their data.
d) MatchDriver - For the most part, Overture lumps singulars and plurals together, and also does a quirky kind of broad matching, alphabetizing the word order of some searches and lumping various searches together. You can do a site search on MatchDriver (or two words, Match Driver) to get a more detailed idea of what they do.
Note that the Google tool referred to in the Sept 2002 thread I link to is the old Google AdWords tool. The current one is much better. I find the AdWords Sandbox very to be useful, but it doesn't have search frequency numbers attached to it. Your best numbers will come from a test PPC campaign.