To turn off certificate checking on your Nokia S60 v3 smartphone like E61, 3250, etc, do the following:

Go to Tools—> App mgr—> Options—> Settings. Change ?software installation? from ?Signed Only? to ?All? and ?Onlline certificate check? to ?Off?.

But remember, this is not recommended as you may unknowingly allow a virus to be installed. So it is recommended you turn on this feature after installing the software