Hello Priyanka,
The Register / Maintain buttons become disabled in two cases. 1) the Hub systems are not configured 2) Runtime objects are not generated.
From the screenshot I can see that hub systems are not configured. You need to configure these from SPRO. There is a shortcut to find the SPRO path--> a. Click on the 'System' (highlighted below)
b) Press on F1(help) c) In the help documentation there is a link to the SPRO path. Now you need to configure the systems here,
Once systems are configured you need to reload the SEGW transaction , then you can see the registered systems under service maintenance node in SEGW.
Then you need to generate the runtime objects(as mentioned in point no. 2 ).
If generation is successful you will be able to register the service.
Please find some helpful links here,
http://help.sap.com/saphelp_gateway20sp06/helpdata/en/cb/5dc700314e4e27be92de2d7065ce8e/content.htm
http://help.sap.com/saphelp_gateway20sp06/helpdata/en/4b/c1553a66274484bdf2a6c04beeeffd/content.htm
Regards.
Atanu