Always include the keywords that people have searched for in the ad title. This makes your charity's ad look like it is a good match to the search terms and has been shown to increase the click through rate in studies.
Do not use keywords repeatedly in the ad as this is against Google's editorial guidelines, but you can use closely related words. This will reinforce the relevance of the ad to searchers.
For example, if someone searches for 'advice cancer' you should include these words in your ad (when I recently did this search, the four cancer charities that came up only included the word cancer, not advice!). If you provide a cancer advisory service, focus on this in the ad, not all the other services that you provide. For example, your ad might say:
Another advantage of including the keywords in the ad is that Google highlights in bold any occurrence of the search terms within the page (as demonstrated above). This will make your charity's ad stand out and will increase the click-through rate.
The main issue with including search keywords in the ad is that you will have to create a new "ad group" for each keyword. Ad groups are a feature of AdWords that let you combine various keywords with an ad. This means you can quickly create one ad for lots of keywords, but stops you from tailoring your ad to your keywords. I would recommend you keep to one set of keywords per ad group.