Emailing a potential PI about applying to a Master's program at their school Is it okay to email professors/PI's to show interest working in their lab when you're applying to the school's Master's program? I saw on another question that unless it is explicitly said, you shouldn't contact the professor directly and should email the admission staff instead. There is a section on PI's webpage with his contact information, but in the Positions section, there are none available currently. Should I not try contacting this person at all?

In terms of the content of the message, I wanted to discuss my interest in a specific field and how working in his lab would help in achieving my goal. Furthermore I wanted to ask for any type of advice which may help when entering graduate school in the field.
Yes, you should contact them.

In terms of the content, you have it half right. Do briefly discuss your interests and why you think you would be a good fit for this lab.  Then simply ask, are you taking graduate students for the next term? 

However, I would say do not ask for advice in general.  That is a bit much and these PIs are busy and won't want to spend the time providing advice for a student they do not know. Keep your email as concise and clear as possible to ensure a response.

Finally, if a professor is a good match for your interests, you should email them even they do not have any openings currently listed on their website. Again, they are busy, they may not have updated their website recently. Or, they may have just received notice that they got a new grant. etc, and so on.  If they are a good match for your interests, it is worth emailing them. 

Good luck.