I do have problem with loading extension into SQL developer.
Ok - so are you going to tell us what 'problem' you are having?
Please post ALL of the info needed to help you:
1. full version of sql developer
2. platform os and version
3. extension name and version
Did you download the extension from this link?
Are you using the extension version appropriate for your sql developer version? Sql Developer 4+ uses the JDeveloper 12.1.3 framework and that framework changed significantly from previous versions. In particular NONE of the extensions written for the previous version will work with sql dev 4; the developers need to modify the extension for the new framework.