Dear gareth,
How exactly did you create the "city" drop down, and where is that setting stored? If it is stored in the standard "City" field of the shipping address, you can use the City variable to check for the selected city (Please note that Name= identifies the name of the rule, not the shipping city!):
Name=Shipping to Alexandra; City=="Alexandra"; Shipping=Weight*25
If you create the dropdown in some other way (with a custom field or so), then things are a bit more complicated, because custom fields and other custom features are not readily available to the plugin.
Regarding the error: Are you sure you have the "25" in the rule? That error message appears when you forget the final term and write "weight*" (without the 25).
Best regards,
Reinhold