Ok, the full solution:
select u.TM_UserID,
u.FullName,
r.LookupValue as Platfrom,
STUFF((
SELECT ', ' + xrs.SkillLookupValue
FROM Reference_Skills xrs
join TM_UserAttributeValue xua on xua.AttributeValue = xrs.SkillLookupValue
WHERE xrs.SkillLookupKey = r.LookupID
and xua.AttributeKeyType = ua.AttributeKeyType
and xua.TM_UserID = u.TM_UserID
FOR XML PATH('')
) ,1,2,'') as skills
from TM_User u
join TM_UserAttributeValue ua on ua.TM_UserID = u.TM_UserID and ua.AttributeKeyType = 'Skill'
join Reference_Skills rs on rs.SkillLookupValue = ua.AttributeValue
join Reference r on r.LookupID = rs.SkillLookupKey
where u.TM_UserID = 106
group by r.LookupID, r.LookupValue, u.TM_UserID, u.FullName, ua.AttributeKeyType