Soapbox Lead Converstion only properly works with Campaigns that are of the Campaign Record Type, "Soapbox Registration," and Campaign Member Type, "Soapbox Registration."
A lot of folks were having problems because they were previously able to select Campaigns with an incorrect Record and/or Member Type.
Because of this, we've adjust the Campaign selector to only offer for selection Campaigns of the correct Record and Member Type.
If you don't see a Campaign there, check that it has the Record and Member Type set to "Soapbox Registration," and is Active.
If you've had to update any Campaigns in this way, you also want to clear the Soapbox site cache, to make sure Soapbox queries Salesforce to grab any newly available Campaigns.