Vcenter 5.5 compatibility with Netbackup 7.5.0.7 ?

yes... even the latest relese Netbackup 7.6.0.1 is also not officially supported for Vcenter 5.5

So is it why I'm not able to add the vcenter  in my NB 7.5.0.7 environnment?

Because Vcenter 5.5 even with NBU 7.5.0.7 is not supported.

Might not be true.... i did see in other discussion where they were able to add and ran into issue when they perfroming the backups.

any error messages?

Alex Davies said the following in October last year:

https://www-secure.symantec.com/connect/forums/vsphere-55-support#comment-9296251

vSphere 5.5 has arrived too late to be supported by NetBackup 7.6 but we expect to support it with a maintenance within the next 6 months. 

Seems support will be added to the next 7.6 patch/release. No support for 7.5 is mentioned.

Best to upgrade to 7.6.0.1 in the meantime and subscribe to this doc:

Statement of Support for NetBackup 7.x in a Virtual Environment:    http://www.symantec.com/docs/TECH127089

You will receive email notification each time this doc is updated.

Some good news!

I have just stumbled upon this comment:

https://www-secure.symantec.com/connect/blogs/what-really-makes-market-leader#comment-9852901

....  NetBackup 7.6 went GA in late December, so we built a special vSphere 5.5 patch for early adopters. The patch is available through our SymBeta download and went live January 31, 2014. If you are an early adopter and very interested, please sign up and we'll be sure you get your hands on it. If you can wait just a bit longer, the upcoming April NetBackup 7.6.0.2 maintenance release will include the patch and be widely available.

Note that vSphere 5.5 will not be supported for NetBackup 7.5.