If you are conditionally admitted, you have until the start of the course / program to register regardless of the issued registration date. When you have met the prerequisites and the final grade for the course has been set, the condition should disappear in Studentportalen.

If the conditionally admitted does not change status automatically, contact antagning@his.se and ask to have the condition removed. You can register on the Studentportalen when the prerequisites are met.
 
If you have been conditionally admitted to a master's program you need to notify admission@his.se when your degree has been issued. You have until the start of the program to prove that prerequisites are met. Remember that when you apply for your Bachelor degree to tell the degree officers that you have applied for further education and need your degree no later than the start of the semester.