I downloaded the NIS 5.0 for Mac Trial version, I installed it and now it has expired. I purchased it from the local market and I have the serial. How can I activate it? The only two selections I see is Purchase (recommended) and Remind me later.
The trial version lets you try the product for a limited time. It's not a retail version, and cannot be activated to provide protection past the trial period.
To protect your computer once the trial is finished, you'd need to purchase and install the retail version of the product.
Since your options are Purchase and "Remind me later" it sounds like you have a Trialware product. Trialware products cannot be activated with a key that you have from another Norton product. I am not sure how you obtained the key that you are trying to use, but the odds are it is not a trialware key. Trailware keys are special to trialware.
Unfortunately the only option right now is to uninstall the trialware product, download the retail product from your Norton account, and install it, and then activate it using the key you already have.