First about sights, I use a Red Dot on my TM-15s and the only way to do that is to use a TM Series RipClip. I use RipClips and have not had any issues. Mil Sim markers are not as easy for me to aim without a red dot, so for me they are a requirement.
As for the Spring, there is a small end and larger end on their springs. You want to make sure the spring "clicks" onto the bolt when you put it on. It will then also "click" into a notch on the inside of the main body when you reinsert the bolt and bolt guide. This will result in the spring and sometimes the bolt as well staying behind in the marker when it is removed. I will use a swab to push the spring out from the front of the marker when necessary.
You should post a pic of your new family member! The TM-15 is my favorite Mil Sim marker that I currently own.
Unofficial Pack Elephant and Gun Whore of the CFLL.
Current Darkside Markers:
Active Reserve: Many, Many More
Retired from Active Service: A few wall hangers that are still 100% functional