How to Configure WebForm to Allow Users to Choose Which CiviCRM Campaign to Contribute to?
You can actually do this without Webform-CiviCRM if you like:


Set up your campaigns in CiviCampaign
Create a new profile to add a field to (or from the Profiles page, click "Fields" beside an existing profile to add a new field there) 
Add a new "Campaign" field to the profile you are working in (this will list all active campaigns in the profile) 
Add the profile to the Contribution Page in question (note: if you added the field to an existing profile it may already be on the Contrib Page)