You can't upgrade via Softaculous from 2.5 because 2.5 cannot be upgraded only migrated to v3.
Just verify the following
1. Your install must be 2.5.4. If it's not 2.5.4, then you need to upgrade to 2.5.4 first.
2. The migrator needs information from your config file, and access to your database. So, you can move your 2.5.4 install into another directory, and use Softaculous to install 3.3 in that directory, then perform the migration via the steps here - http://docs.blesta.com/display/user/Migrating+to+Blesta