RE: Adding item to Product Line Items (Opportunity)
Hi meelamri,
1. First, we have to save Opportunity first, then the (+) button will appear.
Also clear your browser cache and cookies, change another browser to see if it works.
2. Could you provide a screenshot of the properties of the Product Line items?
Click the FORM button on the command bar. In the form editor, click on the Opportunity Products sub-grid and then click the Change Properties button.

We need this screenshot: 
3. Check if there is the "Order Type" field in the Quote and Opportunity form. If there is no "Order Type" field, the (+) button will disappear. Please refer to the last second reply of this post: https://community.dynamics.com/365/f/761/t/298200?pi57428=2#responses

4. What is the version of your D365? This problem is currently known for some D365 trials.
Hope this helps.
Best Regards,
Lu Hao