Excel is a Trademark but it is also a dictionary word what does this mean?
Excel is a registered trademark, now its worth noting that excel is a dictionary word which means that a domain with excel in it does not necessary mean it is infringing on the trademark.
Let me explain, If you use the excel trademark in a domain in the objective to cover Microsoft Products then you are infringing on the trademark, if you objective is to make a brand say for a clothing or a ice cream then again you are infringing on the trademark as its a branding.
Now you would not be infringing on the trademark if you used something to describe an action, since its a dictionary word therefor its impossible to legally own the rights for people not using dictionary words for purposes not describing products, branding or a company figure..
Would not infringe on Microsoft's trademark unless the site was on topic of their products, if it was about life style changes and how to excel in life then your using a dictionary word to describe a non product, therefor no infringement is made.
But I see many sites using the trademark in their domain
This isn't a valid agruement in law, Microsoft own the right to claim against who they like... Just because you see others doing it, doesn't mean you should follow suit, they have infringement claims made against them, they may receive one in time, they may never but if you want to stay on the right side of the law, and retain your rankings then you should never use a infringed domain.
Make a sitename and use the URLS's for targeting products/trademarks
Rather than pointlessly stuffing a keyword in the domain you should opt to use the word excel in created URLS, this will have an better effect... Since Google no longer rewards for domains with keywords in the domain, in fact Google wants people to use branding rather than keyword stuffing.
Come up with a good branding name and build your website into a reputable site to visit, not one that will need require renaming at any time.