There are signed and unsigned applications for the N series phone. Please take note that you will not be able to installed unsigned application to your N series phone. You need to signed the applications before you can installed on your phone. Signed applications will allow you to install on the N series phone without signing it. Some applications will not need to be signed like symbian games(.sis) and can be installed on the N Series phone.
Therefore, if you are unable to installed some software to N Series it is due to the fact the applications is unsigned. Signed applications will have the extension of .sisx